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From: Eric Wong <e@80x24.org>
To: spew@80x24.org
Subject: [PATCH 1/2] thread_pthread.c: eliminate timer thread by restructuring GVL
Date: Thu,  9 Aug 2018 03:46:17 +0000
Message-Id: <20180809034618.20082-1-e@80x24.org>
List-Id: <spew.80x24.org>

This reverts commit 194a6a2c68e9c8a3536b24db18ceac87535a6051 (r64203).

Race conditions which caused the original reversion will be fixed
in the subsequent commit.
---
 internal.h                |   3 +
 process.c                 | 140 +++++++-
 signal.c                  |   7 +-
 test/ruby/test_io.rb      |   9 +-
 test/ruby/test_process.rb |   2 +-
 test/ruby/test_thread.rb  |   5 +-
 thread.c                  | 397 +++++++++++++--------
 thread_pthread.c          | 710 +++++++++++++++++---------------------
 thread_pthread.h          |  20 +-
 thread_win32.c            |  29 +-
 vm_core.h                 |   8 +-
 11 files changed, 746 insertions(+), 584 deletions(-)

diff --git a/internal.h b/internal.h
index 81c556c0be..3b3575b42d 100644
--- a/internal.h
+++ b/internal.h
@@ -77,6 +77,9 @@ extern "C" {
 # define __has_extension __has_feature
 #endif
 
+/* Prevent compiler from reordering access */
+#define ACCESS_ONCE(type,x) (*((volatile type *)&(x)))
+
 #if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L)
 # define STATIC_ASSERT(name, expr) _Static_assert(expr, #name ": " #expr)
 #elif GCC_VERSION_SINCE(4, 6, 0) || __has_extension(c_static_assert)
diff --git a/process.c b/process.c
index 0c534a8a36..43db055cbf 100644
--- a/process.c
+++ b/process.c
@@ -928,6 +928,7 @@ struct waitpid_state {
     int status;
     int options;
     int errnum;
+    int sigwait_fd;
 };
 
 void rb_native_mutex_lock(rb_nativethread_lock_t *);
@@ -936,13 +937,65 @@ void rb_native_cond_signal(rb_nativethread_cond_t *);
 void rb_native_cond_wait(rb_nativethread_cond_t *, rb_nativethread_lock_t *);
 rb_nativethread_cond_t *rb_sleep_cond_get(const rb_execution_context_t *);
 void rb_sleep_cond_put(rb_nativethread_cond_t *);
+int rb_sigwait_fd_get(const rb_thread_t *);
+void rb_sigwait_sleep(const rb_thread_t *, int fd, const struct timespec *);
+void rb_sigwait_fd_put(const rb_thread_t *, int fd);
+
+static int
+sigwait_fd_migrate_signaled_p(struct waitpid_state *w)
+{
+    int signaled = FALSE;
+    rb_thread_t *th = w->ec ? rb_ec_thread_ptr(w->ec) : 0;
+
+    if (th) rb_native_mutex_lock(&th->interrupt_lock);
+
+    if (w->cond) {
+        rb_native_cond_signal(w->cond);
+        signaled = TRUE;
+    }
+
+    if (th) rb_native_mutex_unlock(&th->interrupt_lock);
+
+    return signaled;
+}
+
+/*
+ * When a thread is done using sigwait_fd and there are other threads
+ * sleeping on waitpid, we must kick one of the threads out of
+ * rb_native_cond_wait so it can switch to rb_sigwait_sleep
+ */
+static void
+sigwait_fd_migrate_sleeper(rb_vm_t *vm)
+{
+    struct waitpid_state *w = 0;
+
+    list_for_each(&vm->waiting_pids, w, wnode) {
+        if (sigwait_fd_migrate_signaled_p(w)) return;
+    }
+    list_for_each(&vm->waiting_grps, w, wnode) {
+        if (sigwait_fd_migrate_signaled_p(w)) return;
+    }
+}
+
+void
+rb_sigwait_fd_migrate(rb_vm_t *vm)
+{
+    rb_native_mutex_lock(&vm->waitpid_lock);
+    sigwait_fd_migrate_sleeper(vm);
+    rb_native_mutex_unlock(&vm->waitpid_lock);
+}
 
 static void
 waitpid_notify(struct waitpid_state *w, rb_pid_t ret)
 {
     w->ret = ret;
     list_del_init(&w->wnode);
-    rb_native_cond_signal(w->cond);
+    if (w->cond) {
+        rb_native_cond_signal(w->cond);
+    }
+    else {
+        /* w is owned by this thread */
+    }
 }
 
 #ifdef _WIN32 /* for spawnvp result from mjit.c */
@@ -954,7 +1007,7 @@ waitpid_notify(struct waitpid_state *w, rb_pid_t ret)
 #endif
 
 extern volatile unsigned int ruby_nocldwait; /* signal.c */
-/* called by timer thread */
+/* called by timer thread or thread which acquired sigwait_fd */
 static void
 waitpid_each(struct list_head *head)
 {
@@ -1008,6 +1061,17 @@ waitpid_state_init(struct waitpid_state *w, rb_pid_t pid, int options)
     w->options = options;
 }
 
+static const struct timespec *
+sigwait_sleep_time(void)
+{
+    if (SIGCHLD_LOSSY) {
+        static const struct timespec busy_wait = { 0, 100000000 };
+
+        return &busy_wait;
+    }
+    return 0;
+}
+
 /*
  * must be called with vm->waitpid_lock held, this is not interruptible
  */
@@ -1026,13 +1090,31 @@ ruby_waitpid_locked(rb_vm_t *vm, rb_pid_t pid, int *status, int options,
         if (w.ret == -1) w.errnum = errno;
     }
     else {
-        w.cond = cond;
         w.ec = 0;
+        w.sigwait_fd = -1;
         list_add(w.pid > 0 ? &vm->waiting_pids : &vm->waiting_grps, &w.wnode);
         do {
-            rb_native_cond_wait(w.cond, &vm->waitpid_lock);
+            if (w.sigwait_fd < 0)
+                w.sigwait_fd = rb_sigwait_fd_get(0);
+
+            if (w.sigwait_fd >= 0) {
+                w.cond = 0;
+                rb_native_mutex_unlock(&vm->waitpid_lock);
+                rb_sigwait_sleep(0, w.sigwait_fd, sigwait_sleep_time());
+                rb_native_mutex_lock(&vm->waitpid_lock);
+            }
+            else {
+                w.cond = cond;
+                rb_native_cond_wait(w.cond, &vm->waitpid_lock);
+            }
         } while (!w.ret);
         list_del(&w.wnode);
+
+        /* we're done, maybe other waitpid callers are not: */
+        if (w.sigwait_fd >= 0) {
+            rb_sigwait_fd_put(0, w.sigwait_fd);
+            sigwait_fd_migrate_sleeper(vm);
+        }
     }
     if (status) {
         *status = w.status;
@@ -1047,7 +1129,10 @@ waitpid_wake(void *x)
     struct waitpid_state *w = x;
 
     /* th->interrupt_lock is already held by rb_threadptr_interrupt_common */
-    rb_native_cond_signal(w->cond);
+    if (w->cond)
+        rb_native_cond_signal(w->cond);
+    else
+        rb_thread_wakeup_timer_thread(0); /* kick sigwait_fd */
 }
 
 static void *
@@ -1062,11 +1147,40 @@ waitpid_nogvl(void *x)
      * by the time we enter this.  And we may also be interrupted.
      */
     if (!w->ret && !RUBY_VM_INTERRUPTED_ANY(w->ec)) {
-        if (SIGCHLD_LOSSY) {
-            rb_thread_wakeup_timer_thread();
+        if (w->sigwait_fd < 0)
+            w->sigwait_fd = rb_sigwait_fd_get(th);
+
+        if (w->sigwait_fd >= 0) {
+            rb_nativethread_cond_t *cond = w->cond;
+
+            w->cond = 0;
+            rb_native_mutex_unlock(&th->interrupt_lock);
+            rb_sigwait_sleep(th, w->sigwait_fd, sigwait_sleep_time());
+            rb_native_mutex_lock(&th->interrupt_lock);
+            w->cond = cond;
         }
-        rb_native_cond_wait(w->cond, &th->interrupt_lock);
+        else {
+            if (!w->cond)
+                w->cond = rb_sleep_cond_get(w->ec);
+
+            /* another thread calling rb_sigwait_sleep will process
+             * signals for us */
+            if (SIGCHLD_LOSSY) {
+                rb_thread_wakeup_timer_thread(0);
+            }
+            rb_native_cond_wait(w->cond, &th->interrupt_lock);
+        }
+    }
+
+    /*
+     * we must release th->native_thread_data.sleep_cond when
+     * re-acquiring GVL:
+     */
+    if (w->cond) {
+        rb_sleep_cond_put(w->cond);
+        w->cond = 0;
     }
+
     rb_native_mutex_unlock(&th->interrupt_lock);
 
     return 0;
@@ -1096,8 +1210,15 @@ waitpid_cleanup(VALUE x)
         list_del(&w->wnode);
         rb_native_mutex_unlock(&vm->waitpid_lock);
     }
-    rb_sleep_cond_put(w->cond);
 
+    /* we may have never released and re-acquired GVL */
+    if (w->cond)
+        rb_sleep_cond_put(w->cond);
+
+    if (w->sigwait_fd >= 0) {
+        rb_sigwait_fd_put(rb_ec_thread_ptr(w->ec), w->sigwait_fd);
+        rb_sigwait_fd_migrate(rb_ec_vm_ptr(w->ec));
+    }
     return Qfalse;
 }
 
@@ -1124,6 +1245,7 @@ waitpid_wait(struct waitpid_state *w)
     }
     else {
         w->cond = rb_sleep_cond_get(w->ec);
+        w->sigwait_fd = -1;
         /* order matters, favor specified PIDs rather than -1 or 0 */
         list_add(w->pid > 0 ? &vm->waiting_pids : &vm->waiting_grps, &w->wnode);
     }
diff --git a/signal.c b/signal.c
index 6393273adf..65b45cf6bd 100644
--- a/signal.c
+++ b/signal.c
@@ -709,9 +709,6 @@ signal_enque(int sig)
 
 static rb_atomic_t sigchld_hit;
 
-/* Prevent compiler from reordering access */
-#define ACCESS_ONCE(type,x) (*((volatile type *)&(x)))
-
 static RETSIGTYPE
 sighandler(int sig)
 {
@@ -730,7 +727,7 @@ sighandler(int sig)
     else {
         signal_enque(sig);
     }
-    rb_thread_wakeup_timer_thread();
+    rb_thread_wakeup_timer_thread(sig);
 #if !defined(BSD_SIGNAL) && !defined(POSIX_SIGNAL)
     ruby_signal(sig, sighandler);
 #endif
@@ -764,7 +761,6 @@ rb_enable_interrupt(void)
 #ifdef HAVE_PTHREAD_SIGMASK
     sigset_t mask;
     sigemptyset(&mask);
-    sigaddset(&mask, RUBY_SIGCHLD); /* timer-thread handles this */
     pthread_sigmask(SIG_SETMASK, &mask, NULL);
 #endif
 }
@@ -1077,7 +1073,6 @@ rb_trap_exit(void)
 
 void ruby_waitpid_all(rb_vm_t *); /* process.c */
 
-/* only runs in the timer-thread */
 void
 ruby_sigchld_handler(rb_vm_t *vm)
 {
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index a1462c53b2..d71efeb8a2 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -3556,7 +3556,8 @@ def test_open_flag_binary
   end if File::BINARY != 0
 
   def test_race_gets_and_close
-    assert_separately([], "#{<<-"begin;"}\n#{<<-"end;"}")
+    opt = { signal: :ABRT, timeout: 200 }
+    assert_separately([], "#{<<-"begin;"}\n#{<<-"end;"}", opt)
     bug13076 = '[ruby-core:78845] [Bug #13076]'
     begin;
       10.times do |i|
@@ -3578,9 +3579,9 @@ def test_race_gets_and_close
           w.close
           r.close
         end
-        assert_nothing_raised(IOError, bug13076) {
-          t.each(&:join)
-        }
+        t.each do |th|
+          assert_same(th, th.join(2), bug13076)
+        end
       end
     end;
   end
diff --git a/test/ruby/test_process.rb b/test/ruby/test_process.rb
index 759230f834..a0b08dd110 100644
--- a/test/ruby/test_process.rb
+++ b/test/ruby/test_process.rb
@@ -1767,7 +1767,7 @@ def test_daemon_no_threads
           puts Dir.entries("/proc/self/task") - %W[. ..]
         end
         bug4920 = '[ruby-dev:43873]'
-        assert_equal(2, data.size, bug4920)
+        assert_include(1..2, data.size, bug4920)
         assert_not_include(data.map(&:to_i), pid)
       end
     else # darwin
diff --git a/test/ruby/test_thread.rb b/test/ruby/test_thread.rb
index 03fd1e3075..56a7bfb23b 100644
--- a/test/ruby/test_thread.rb
+++ b/test/ruby/test_thread.rb
@@ -952,15 +952,16 @@ def test_backtrace
   def test_thread_timer_and_interrupt
     bug5757 = '[ruby-dev:44985]'
     pid = nil
-    cmd = 'Signal.trap(:INT, "DEFAULT"); r,=IO.pipe; Thread.start {Thread.pass until Thread.main.stop?; puts; STDOUT.flush}; r.read'
+    cmd = 'Signal.trap(:INT, "DEFAULT"); pipe=IO.pipe; Thread.start {Thread.pass until Thread.main.stop?; puts; STDOUT.flush}; pipe[0].read'
     opt = {}
     opt[:new_pgroup] = true if /mswin|mingw/ =~ RUBY_PLATFORM
     s, t, _err = EnvUtil.invoke_ruby(['-e', cmd], "", true, true, opt) do |in_p, out_p, err_p, cpid|
+      assert IO.select([out_p], nil, nil, 10), 'subprocess not ready'
       out_p.gets
       pid = cpid
       t0 = Time.now.to_f
       Process.kill(:SIGINT, pid)
-      Process.wait(pid)
+      Timeout.timeout(10) { Process.wait(pid) }
       t1 = Time.now.to_f
       [$?, t1 - t0, err_p.read]
     end
diff --git a/thread.c b/thread.c
index a7d48a464f..9c7fcee05f 100644
--- a/thread.c
+++ b/thread.c
@@ -106,8 +106,13 @@ static int rb_threadptr_pending_interrupt_empty_p(const rb_thread_t *th);
 static const char *thread_status_name(rb_thread_t *th, int detail);
 static void timespec_add(struct timespec *, const struct timespec *);
 static void timespec_sub(struct timespec *, const struct timespec *);
+static int timespec_cmp(const struct timespec *a, const struct timespec *b);
 static int timespec_update_expire(struct timespec *, const struct timespec *);
 static void getclockofday(struct timespec *);
+NORETURN(static void async_bug_fd(const char *mesg, int errno_arg, int fd));
+static int consume_communication_pipe(int fd);
+static int check_signals_nogvl(rb_thread_t *, int sigwait_fd);
+void rb_sigwait_fd_migrate(rb_vm_t *); /* process.c */
 
 #define eKillSignal INT2FIX(0)
 #define eTerminateSignal INT2FIX(1)
@@ -348,7 +353,14 @@ rb_thread_s_debug_set(VALUE self, VALUE val)
 #endif
 NOINLINE(static int thread_start_func_2(rb_thread_t *th, VALUE *stack_start,
 					VALUE *register_stack_start));
-static void timer_thread_function(void *);
+static void timer_thread_function(void);
+void ruby_sigchld_handler(rb_vm_t *); /* signal.c */
+
+static void
+ubf_sigwait(void *ignore)
+{
+    rb_thread_wakeup_timer_thread(0);
+}
 
 #if   defined(_WIN32)
 #include "thread_win32.c"
@@ -373,6 +385,15 @@ static void timer_thread_function(void *);
 #error "unsupported thread type"
 #endif
 
+/*
+ * TODO: somebody with win32 knowledge should be able to get rid of
+ * timer-thread by busy-waiting on signals.  And it should be possible
+ * to make the GVL in thread_pthread.c be platform-independent.
+ */
+#ifndef BUSY_WAIT_SIGNALS
+#  define BUSY_WAIT_SIGNALS (0)
+#endif
+
 #if THREAD_DEBUG
 static int debug_mutex_initialized = 1;
 static rb_nativethread_lock_t debug_mutex;
@@ -412,7 +433,6 @@ rb_vm_gvl_destroy(rb_vm_t *vm)
 {
     gvl_release(vm);
     gvl_destroy(vm);
-    rb_native_mutex_destroy(&vm->thread_destruct_lock);
     if (0) {
         /* may be held by running threads */
         rb_native_mutex_destroy(&vm->waitpid_lock);
@@ -773,10 +793,6 @@ thread_start_func_2(rb_thread_t *th, VALUE *stack_start, VALUE *register_stack_s
 
 	rb_fiber_close(th->ec->fiber_ptr);
     }
-    rb_native_mutex_lock(&th->vm->thread_destruct_lock);
-    /* make sure vm->running_thread never point me after this point.*/
-    th->vm->running_thread = NULL;
-    rb_native_mutex_unlock(&th->vm->thread_destruct_lock);
     thread_cleanup_func(th, FALSE);
     gvl_release(th->vm);
 
@@ -2163,6 +2179,14 @@ rb_threadptr_execute_interrupts(rb_thread_t *th, int blocking_timing)
 	/* signal handling */
 	if (trap_interrupt && (th == th->vm->main_thread)) {
 	    enum rb_thread_status prev_status = th->status;
+	    int sigwait_fd = rb_sigwait_fd_get(th);
+
+	    if (sigwait_fd >= 0) {
+		(void)consume_communication_pipe(sigwait_fd);
+		ruby_sigchld_handler(th->vm);
+		rb_sigwait_fd_put(th, sigwait_fd);
+		rb_sigwait_fd_migrate(th->vm);
+	    }
 	    th->status = THREAD_RUNNABLE;
 	    while ((sig = rb_get_next_signal()) != 0) {
 		rb_signal_exec(th, sig);
@@ -3840,86 +3864,95 @@ wait_retryable(int *result, int errnum, struct timespec *timeout,
     return FALSE;
 }
 
-#define restore_fdset(fds1, fds2) \
-    ((fds1) ? rb_fd_dup(fds1, fds2) : (void)0)
-
 struct select_set {
-    rb_fdset_t read;
-    rb_fdset_t write;
-    rb_fdset_t except;
+    int max;
+    int sigwait_fd;
+    rb_thread_t *th;
+    rb_fdset_t *rset;
+    rb_fdset_t *wset;
+    rb_fdset_t *eset;
+    rb_fdset_t orig_rset;
+    rb_fdset_t orig_wset;
+    rb_fdset_t orig_eset;
+    struct timeval *timeout;
 };
 
-static size_t
-select_set_memsize(const void *p)
+static VALUE
+select_set_free(VALUE p)
 {
-    return sizeof(struct select_set);
+    struct select_set *set = (struct select_set *)p;
+
+    if (set->sigwait_fd >= 0) {
+        rb_sigwait_fd_put(set->th, set->sigwait_fd);
+        rb_sigwait_fd_migrate(set->th->vm);
+    }
+
+    rb_fd_term(&set->orig_rset);
+    rb_fd_term(&set->orig_wset);
+    rb_fd_term(&set->orig_eset);
+
+    return Qfalse;
 }
 
-static void
-select_set_free(void *p)
+static const struct timespec *
+sigwait_timeout(rb_thread_t *th, int sigwait_fd, const struct timespec *orig,
+                int *drained_p)
 {
-    struct select_set *orig = p;
+    static const struct timespec quantum = { 0, TIME_QUANTUM_USEC * 1000 };
 
-    rb_fd_term(&orig->read);
-    rb_fd_term(&orig->write);
-    rb_fd_term(&orig->except);
-    xfree(orig);
-}
+    if (sigwait_fd >= 0 && (!ubf_threads_empty() || BUSY_WAIT_SIGNALS)) {
+        *drained_p = check_signals_nogvl(th, sigwait_fd);
+        if (!orig || timespec_cmp(orig, &quantum) > 0)
+            return &quantum;
+    }
 
-static const rb_data_type_t select_set_type = {
-    "select_set",
-    {NULL, select_set_free, select_set_memsize,},
-    0, 0, RUBY_TYPED_FREE_IMMEDIATELY
-};
+    return orig;
+}
 
-static int
-do_select(int n, rb_fdset_t *const readfds, rb_fdset_t *const writefds,
-	  rb_fdset_t *const exceptfds, struct timeval *timeout)
+static VALUE
+do_select(VALUE p)
 {
+    struct select_set *set = (struct select_set *)p;
     int MAYBE_UNUSED(result);
     int lerrno;
     struct timespec ts, end, *tsp;
-    rb_thread_t *th = GET_THREAD();
-    VALUE o;
-    struct select_set *orig;
-
-    o = TypedData_Make_Struct(0, struct select_set, &select_set_type, orig);
+    const struct timespec *to;
+    struct timeval tv;
 
-    timeout_prepare(&tsp, &ts, &end, timeout);
+    timeout_prepare(&tsp, &ts, &end, set->timeout);
+#define restore_fdset(dst, src) \
+    ((dst) ? rb_fd_dup(dst, src) : (void)0)
 #define do_select_update() \
-    (restore_fdset(readfds, &orig->read), \
-     restore_fdset(writefds, &orig->write), \
-     restore_fdset(exceptfds, &orig->except), \
+    (restore_fdset(set->rset, &set->orig_rset), \
+     restore_fdset(set->wset, &set->orig_wset), \
+     restore_fdset(set->eset, &set->orig_eset), \
      TRUE)
 
-#define fd_init_copy(f) \
-    (f##fds) ? rb_fd_init_copy(&orig->f, f##fds) : rb_fd_no_init(&orig->f)
-    fd_init_copy(read);
-    fd_init_copy(write);
-    fd_init_copy(except);
-#undef fd_init_copy
-
     do {
+	int drained;
 	lerrno = 0;
 
-	BLOCKING_REGION(th, {
-	    result = native_fd_select(n, readfds, writefds, exceptfds,
-				      timeval_for(timeout, tsp), th);
+	BLOCKING_REGION(set->th, {
+	    to = sigwait_timeout(set->th, set->sigwait_fd, tsp, &drained);
+	    result = native_fd_select(set->max, set->rset, set->wset, set->eset,
+				      timeval_for(&tv, to), set->th);
 	    if (result < 0) lerrno = errno;
-	}, ubf_select, th, FALSE);
+	}, set->sigwait_fd >= 0 ? ubf_sigwait : ubf_select, set->th, FALSE);
 
-	RUBY_VM_CHECK_INTS_BLOCKING(th->ec); /* may raise */
-    } while (wait_retryable(&result, lerrno, tsp, &end) && do_select_update());
+        if (set->sigwait_fd >= 0) {
+            if (result > 0 && rb_fd_isset(set->sigwait_fd, set->rset))
+                result--;
+            (void)check_signals_nogvl(set->th, set->sigwait_fd);
+        }
 
-    /* didn't raise, perform cleanup ourselves */
-    select_set_free(orig);
-    rb_gc_force_recycle(o);
+        RUBY_VM_CHECK_INTS_BLOCKING(set->th->ec); /* may raise */
+    } while (wait_retryable(&result, lerrno, tsp, &end) && do_select_update());
 
     if (result < 0) {
 	errno = lerrno;
     }
 
-    return result;
+    return (VALUE)result;
 }
 
 static void
@@ -3955,11 +3988,42 @@ rb_thread_fd_writable(int fd)
     return TRUE;
 }
 
+static rb_fdset_t *
+init_set_fd(int fd, rb_fdset_t *fds)
+{
+    if (fd < 0) {
+	return 0;
+    }
+    rb_fd_init(fds);
+    rb_fd_set(fd, fds);
+
+    return fds;
+}
+
 int
 rb_thread_fd_select(int max, rb_fdset_t * read, rb_fdset_t * write, rb_fdset_t * except,
 		    struct timeval *timeout)
 {
-    if (!read && !write && !except) {
+    struct select_set set;
+
+    set.th = GET_THREAD();
+    set.max = max;
+    set.sigwait_fd = rb_sigwait_fd_get(set.th);
+    set.rset = read;
+    set.wset = write;
+    set.eset = except;
+    set.timeout = timeout;
+
+    if (set.sigwait_fd >= 0) {
+        if (set.rset)
+            rb_fd_set(set.sigwait_fd, set.rset);
+        else
+            set.rset = init_set_fd(set.sigwait_fd, &set.orig_rset);
+        if (set.sigwait_fd > set.max) {
+            set.max = set.sigwait_fd + 1;
+        }
+    }
+    if (!set.rset && !set.wset && !set.eset) {
 	if (!timeout) {
 	    rb_thread_sleep_forever();
 	    return 0;
@@ -3968,16 +4032,23 @@ rb_thread_fd_select(int max, rb_fdset_t * read, rb_fdset_t * write, rb_fdset_t *
 	return 0;
     }
 
-    if (read) {
-	rb_fd_resize(max - 1, read);
-    }
-    if (write) {
-	rb_fd_resize(max - 1, write);
-    }
-    if (except) {
-	rb_fd_resize(max - 1, except);
-    }
-    return do_select(max, read, write, except, timeout);
+#define fd_init_copy(f) do { \
+        if (set.f) { \
+            rb_fd_resize(set.max - 1, set.f); \
+            if (&set.orig_##f != set.f) { /* sigwait_fd */ \
+                rb_fd_init_copy(&set.orig_##f, set.f); \
+            } \
+        } \
+        else { \
+            rb_fd_no_init(&set.orig_##f); \
+        } \
+    } while (0)
+    fd_init_copy(rset);
+    fd_init_copy(wset);
+    fd_init_copy(eset);
+#undef fd_init_copy
+
+    return (int)rb_ensure(do_select, (VALUE)&set, select_set_free, (VALUE)&set);
 }
 
 #ifdef USE_POLL
@@ -3991,68 +4062,64 @@ rb_thread_fd_select(int max, rb_fdset_t * read, rb_fdset_t * write, rb_fdset_t *
 #  define POLLERR_SET (0)
 #endif
 
-#ifndef HAVE_PPOLL
-/* TODO: don't ignore sigmask */
-static int
-ruby_ppoll(struct pollfd *fds, nfds_t nfds,
-      const struct timespec *ts, const sigset_t *sigmask)
-{
-    int timeout_ms;
-
-    if (ts) {
-	int tmp, tmp2;
-
-	if (ts->tv_sec > INT_MAX/1000)
-	    timeout_ms = INT_MAX;
-	else {
-	    tmp = (int)(ts->tv_sec * 1000);
-	    /* round up 1ns to 1ms to avoid excessive wakeups for <1ms sleep */
-	    tmp2 = (int)((ts->tv_nsec + 999999L) / (1000L * 1000L));
-	    if (INT_MAX - tmp < tmp2)
-		timeout_ms = INT_MAX;
-	    else
-		timeout_ms = (int)(tmp + tmp2);
-	}
-    }
-    else
-	timeout_ms = -1;
-
-    return poll(fds, nfds, timeout_ms);
-}
-#  define ppoll(fds,nfds,ts,sigmask) ruby_ppoll((fds),(nfds),(ts),(sigmask))
-#endif
-
 /*
  * returns a mask of events
  */
 int
 rb_wait_for_single_fd(int fd, int events, struct timeval *timeout)
 {
-    struct pollfd fds;
+    struct pollfd fds[2];
     int result = 0, lerrno;
     struct timespec ts, end, *tsp;
+    const struct timespec *to;
+    int drained;
     rb_thread_t *th = GET_THREAD();
+    nfds_t nfds;
+    rb_unblock_function_t *ubf;
 
     timeout_prepare(&tsp, &ts, &end, timeout);
-    fds.fd = fd;
-    fds.events = (short)events;
-
+    fds[0].fd = fd;
+    fds[0].events = (short)events;
     do {
-        fds.revents = 0;
+        fds[0].revents = 0;
+        fds[1].fd = rb_sigwait_fd_get(th);
+
+        if (fds[1].fd >= 0) {
+            fds[1].events = POLLIN;
+            fds[1].revents = 0;
+            nfds = 2;
+            ubf = ubf_sigwait;
+        }
+        else {
+            nfds = 1;
+            ubf = ubf_select;
+        }
+
         lerrno = 0;
         BLOCKING_REGION(th, {
-            result = ppoll(&fds, 1, tsp, NULL);
+            to = sigwait_timeout(th, fds[1].fd, tsp, &drained);
+            result = ppoll(fds, nfds, to, NULL);
             if (result < 0) lerrno = errno;
-        }, ubf_select, th, FALSE);
+        }, ubf, th, FALSE);
 
+        if (fds[1].fd >= 0) {
+            if (result > 0 && fds[1].revents) {
+                result--;
+                fds[1].revents = 0;
+            }
+            (void)check_signals_nogvl(th, fds[1].fd);
+            rb_sigwait_fd_put(th, fds[1].fd);
+            rb_sigwait_fd_migrate(th->vm);
+        }
         RUBY_VM_CHECK_INTS_BLOCKING(th->ec);
     } while (wait_retryable(&result, lerrno, tsp, &end));
+
     if (result < 0) {
 	errno = lerrno;
 	return -1;
     }
 
-    if (fds.revents & POLLNVAL) {
+    if (fds[0].revents & POLLNVAL) {
 	errno = EBADF;
 	return -1;
     }
@@ -4062,32 +4129,20 @@ rb_wait_for_single_fd(int fd, int events, struct timeval *timeout)
      * Therefore we need to fix it up.
      */
     result = 0;
-    if (fds.revents & POLLIN_SET)
+    if (fds[0].revents & POLLIN_SET)
 	result |= RB_WAITFD_IN;
-    if (fds.revents & POLLOUT_SET)
+    if (fds[0].revents & POLLOUT_SET)
 	result |= RB_WAITFD_OUT;
-    if (fds.revents & POLLEX_SET)
+    if (fds[0].revents & POLLEX_SET)
 	result |= RB_WAITFD_PRI;
 
     /* all requested events are ready if there is an error */
-    if (fds.revents & POLLERR_SET)
+    if (fds[0].revents & POLLERR_SET)
 	result |= events;
 
     return result;
 }
 #else /* ! USE_POLL - implement rb_io_poll_fd() using select() */
-static rb_fdset_t *
-init_set_fd(int fd, rb_fdset_t *fds)
-{
-    if (fd < 0) {
-	return 0;
-    }
-    rb_fd_init(fds);
-    rb_fd_set(fd, fds);
-
-    return fds;
-}
-
 struct select_args {
     union {
 	int fd;
@@ -4168,10 +4223,6 @@ rb_gc_set_stack_end(VALUE **stack_end_p)
 }
 #endif
 
-
-/* signal.c */
-void ruby_sigchld_handler(rb_vm_t *);
-
 /*
  *
  */
@@ -4187,36 +4238,81 @@ rb_threadptr_check_signal(rb_thread_t *mth)
 }
 
 static void
-timer_thread_function(void *arg)
+timer_thread_function(void)
 {
-    rb_vm_t *vm = GET_VM(); /* TODO: fix me for Multi-VM */
+    volatile rb_execution_context_t *ec;
 
-    /*
-     * Tricky: thread_destruct_lock doesn't close a race against
-     * vm->running_thread switch. however it guarantees th->running_thread
-     * point to valid pointer or NULL.
-     */
-    rb_native_mutex_lock(&vm->thread_destruct_lock);
     /* for time slice */
-    if (vm->running_thread) {
-	RUBY_VM_SET_TIMER_INTERRUPT(vm->running_thread->ec);
-    }
-    rb_native_mutex_unlock(&vm->thread_destruct_lock);
-
-    /* check signal */
-    ruby_sigchld_handler(vm);
-    rb_threadptr_check_signal(vm->main_thread);
+    ec = ACCESS_ONCE(rb_execution_context_t *,
+                     ruby_current_execution_context_ptr);
+    if (ec) RUBY_VM_SET_TIMER_INTERRUPT(ec);
+}
 
-#if 0
-    /* prove profiler */
-    if (vm->prove_profile.enable) {
-	rb_thread_t *th = vm->running_thread;
+static void
+async_bug_fd(const char *mesg, int errno_arg, int fd)
+{
+    char buff[64];
+    size_t n = strlcpy(buff, mesg, sizeof(buff));
+    if (n < sizeof(buff)-3) {
+	ruby_snprintf(buff+n, sizeof(buff)-n, "(%d)", fd);
+    }
+    rb_async_bug_errno(buff, errno_arg);
+}
 
-	if (vm->during_gc) {
-	    /* GC prove profiling */
+/* VM-dependent API is not available for this function */
+static int
+consume_communication_pipe(int fd)
+{
+#define CCP_READ_BUFF_SIZE 1024
+    /* buffer can be shared because no one refers to them. */
+    static char buff[CCP_READ_BUFF_SIZE];
+    ssize_t result;
+    int ret = FALSE; /* for rb_sigwait_sleep */
+
+    while (1) {
+	result = read(fd, buff, sizeof(buff));
+	if (result > 0) {
+	    ret = TRUE;
+	    if (result < (ssize_t)sizeof(buff)) {
+		return ret;
+	    }
 	}
-    }
+	else if (result == 0) {
+	    return ret;
+	}
+	else if (result < 0) {
+	    int e = errno;
+	    switch (e) {
+	      case EINTR:
+		continue; /* retry */
+	      case EAGAIN:
+#if defined(EWOULDBLOCK) && EWOULDBLOCK != EAGAIN
+	      case EWOULDBLOCK:
 #endif
+		return ret;
+	      default:
+		async_bug_fd("consume_communication_pipe: read", e, fd);
+	    }
+	}
+    }
+}
+
+static int
+check_signals_nogvl(rb_thread_t *th, int sigwait_fd)
+{
+    rb_vm_t *vm = GET_VM(); /* th may be 0 */
+    int ret = consume_communication_pipe(sigwait_fd);
+    ubf_wakeup_all_threads();
+    ruby_sigchld_handler(vm);
+    if (rb_signal_buff_size()) {
+        if (th == vm->main_thread)
+            /* no need to lock + wakeup if already in main thread */
+            RUBY_VM_SET_TRAP_INTERRUPT(th->ec);
+        else
+            threadptr_trap_interrupt(vm->main_thread);
+        ret = TRUE; /* for SIGCHLD_LOSSY && rb_sigwait_sleep */
+    }
+    return ret;
 }
 
 void
@@ -5046,7 +5142,6 @@ Init_Thread(void)
 	    /* acquire global vm lock */
 	    gvl_init(th->vm);
 	    gvl_acquire(th->vm, th);
-            rb_native_mutex_initialize(&th->vm->thread_destruct_lock);
             rb_native_mutex_initialize(&th->vm->waitpid_lock);
             rb_native_mutex_initialize(&th->interrupt_lock);
 
diff --git a/thread_pthread.c b/thread_pthread.c
index 29805ef2df..545cc2fa3b 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -45,27 +45,21 @@ void rb_native_cond_broadcast(rb_nativethread_cond_t *cond);
 void rb_native_cond_wait(rb_nativethread_cond_t *cond, rb_nativethread_lock_t *mutex);
 void rb_native_cond_initialize(rb_nativethread_cond_t *cond);
 void rb_native_cond_destroy(rb_nativethread_cond_t *cond);
-static void rb_thread_wakeup_timer_thread_low(void);
 static void clear_thread_cache_altstack(void);
+static void ubf_wakeup_all_threads(void);
+static int ubf_threads_empty(void);
+static int native_cond_timedwait(rb_nativethread_cond_t *, pthread_mutex_t *,
+                                 const struct timespec *);
+static const struct timespec *sigwait_timeout(rb_thread_t *, int sigwait_fd,
+                                              const struct timespec *,
+                                              int *drained_p);
 
-#define TIMER_THREAD_MASK    (1)
-#define TIMER_THREAD_SLEEPY  (2|TIMER_THREAD_MASK)
-#define TIMER_THREAD_BUSY    (4|TIMER_THREAD_MASK)
+#define TIMER_THREAD_CREATED_P() (timer_thread_pipe.owner_process == getpid())
 
-#if defined(HAVE_POLL) && defined(HAVE_FCNTL) && defined(F_GETFL) && \
-   defined(F_SETFL) && defined(O_NONBLOCK) && \
-   defined(F_GETFD) && defined(F_SETFD) && defined(FD_CLOEXEC)
-/* The timer thread sleeps while only one Ruby thread is running. */
-# define TIMER_IMPL TIMER_THREAD_SLEEPY
-#else
-# define TIMER_IMPL TIMER_THREAD_BUSY
-#endif
-
-static struct {
-    pthread_t id;
-    int created;
-} timer_thread;
-#define TIMER_THREAD_CREATED_P() (timer_thread.created != 0)
+/* for testing, and in case we come across a platform w/o pipes: */
+#define BUSY_WAIT_SIGNALS (0)
+#define THREAD_INVALID ((const rb_thread_t *)-1)
+static const rb_thread_t *sigwait_th;
 
 #ifdef HAVE_SCHED_YIELD
 #define native_thread_yield() (void)sched_yield()
@@ -82,49 +76,96 @@ static pthread_condattr_t *condattr_monotonic = &condattr_mono;
 static const void *const condattr_monotonic = NULL;
 #endif
 
+/* 100ms.  10ms is too small for user level thread scheduling
+ * on recent Linux (tested on 2.6.35)
+ */
+#define TIME_QUANTUM_USEC (100 * 1000)
+
+static struct timespec native_cond_timeout(rb_nativethread_cond_t *,
+                                           struct timespec rel);
+
 static void
-gvl_acquire_common(rb_vm_t *vm)
+gvl_acquire_common(rb_vm_t *vm, rb_thread_t *th)
 {
     if (vm->gvl.acquired) {
-
-	if (!vm->gvl.waiting++) {
-	    /*
-	     * Wake up timer thread iff timer thread is slept.
-	     * When timer thread is polling mode, we don't want to
-	     * make confusing timer thread interval time.
-	     */
-	    rb_thread_wakeup_timer_thread_low();
-	}
-
-	while (vm->gvl.acquired) {
-            rb_native_cond_wait(&vm->gvl.cond, &vm->gvl.lock);
-	}
-
-	--vm->gvl.waiting;
-
-	if (vm->gvl.need_yield) {
-	    vm->gvl.need_yield = 0;
+        native_thread_data_t *nd = &th->native_thread_data;
+
+        VM_ASSERT(th->unblock.func == 0 && "we reuse ubf_list for GVL waitq");
+
+        list_add_tail(&vm->gvl.waitq, &nd->ubf_list);
+        do {
+            if (!vm->gvl.timer) {
+                static struct timespec ts;
+                static int err = ETIMEDOUT;
+
+                /*
+                 * become designated timer thread to kick vm->gvl.acquired
+                 * periodically.  Continue on old timeout if it expired:
+                 */
+                if (err == ETIMEDOUT) {
+                    ts.tv_sec = 0;
+                    ts.tv_nsec = TIME_QUANTUM_USEC * 1000;
+                    ts = native_cond_timeout(&nd->cond.gvlq, ts);
+                }
+                vm->gvl.timer = th;
+                err = native_cond_timedwait(&nd->cond.gvlq, &vm->gvl.lock, &ts);
+                vm->gvl.timer = 0;
+                ubf_wakeup_all_threads();
+
+                /*
+                 * Timeslice.  We can't touch thread_destruct_lock here,
+                 * as the process may fork while this thread is contending
+                 * for GVL:
+                 */
+                if (vm->gvl.acquired) timer_thread_function();
+            }
+            else {
+                rb_native_cond_wait(&nd->cond.gvlq, &vm->gvl.lock);
+            }
+        } while (vm->gvl.acquired);
+
+        list_del_init(&nd->ubf_list);
+
+        if (vm->gvl.need_yield) {
+            vm->gvl.need_yield = 0;
             rb_native_cond_signal(&vm->gvl.switch_cond);
-	}
+        }
     }
+    vm->gvl.acquired = th;
+    /*
+     * Designate the next gvl.timer thread, favor the last thread in
+     * the waitq since it will be in waitq longest
+     */
+    if (!vm->gvl.timer) {
+        native_thread_data_t *last;
 
-    vm->gvl.acquired = 1;
+        last = list_tail(&vm->gvl.waitq, native_thread_data_t, ubf_list);
+        if (last) {
+            rb_native_cond_signal(&last->cond.gvlq);
+        }
+        else if (!ubf_threads_empty()) {
+            rb_thread_wakeup_timer_thread(0);
+        }
+    }
 }
 
 static void
 gvl_acquire(rb_vm_t *vm, rb_thread_t *th)
 {
     rb_native_mutex_lock(&vm->gvl.lock);
-    gvl_acquire_common(vm);
+    gvl_acquire_common(vm, th);
     rb_native_mutex_unlock(&vm->gvl.lock);
 }
 
-static void
+static native_thread_data_t *
 gvl_release_common(rb_vm_t *vm)
 {
+    native_thread_data_t *next;
     vm->gvl.acquired = 0;
-    if (vm->gvl.waiting > 0)
-        rb_native_cond_signal(&vm->gvl.cond);
+    next = list_top(&vm->gvl.waitq, native_thread_data_t, ubf_list);
+    if (next) rb_native_cond_signal(&next->cond.gvlq);
+
+    return next;
 }
 
 static void
@@ -138,34 +179,38 @@ gvl_release(rb_vm_t *vm)
 static void
 gvl_yield(rb_vm_t *vm, rb_thread_t *th)
 {
-    rb_native_mutex_lock(&vm->gvl.lock);
+    native_thread_data_t *next;
 
-    gvl_release_common(vm);
+    rb_native_mutex_lock(&vm->gvl.lock);
+    next = gvl_release_common(vm);
 
     /* An another thread is processing GVL yield. */
     if (UNLIKELY(vm->gvl.wait_yield)) {
-	while (vm->gvl.wait_yield)
+        while (vm->gvl.wait_yield)
             rb_native_cond_wait(&vm->gvl.switch_wait_cond, &vm->gvl.lock);
-	goto acquire;
     }
-
-    if (vm->gvl.waiting > 0) {
-	/* Wait until another thread task take GVL. */
-	vm->gvl.need_yield = 1;
-	vm->gvl.wait_yield = 1;
-	while (vm->gvl.need_yield)
+    else if (next) {
+        /* Wait until another thread task takes GVL. */
+        vm->gvl.need_yield = 1;
+        vm->gvl.wait_yield = 1;
+        while (vm->gvl.need_yield)
             rb_native_cond_wait(&vm->gvl.switch_cond, &vm->gvl.lock);
-	vm->gvl.wait_yield = 0;
+        vm->gvl.wait_yield = 0;
+        rb_native_cond_broadcast(&vm->gvl.switch_wait_cond);
     }
     else {
-	rb_native_mutex_unlock(&vm->gvl.lock);
-	sched_yield();
+        rb_native_mutex_unlock(&vm->gvl.lock);
+        /*
+         * GVL was not contended when we released, so we have no potential
+         * contenders for reacquisition.  Perhaps they are stuck in blocking
+         * region w/o GVL, too, so we kick them:
+         */
+        ubf_wakeup_all_threads();
+        native_thread_yield();
         rb_native_mutex_lock(&vm->gvl.lock);
+        rb_native_cond_broadcast(&vm->gvl.switch_wait_cond);
     }
-
-    rb_native_cond_broadcast(&vm->gvl.switch_wait_cond);
-  acquire:
-    gvl_acquire_common(vm);
+    gvl_acquire_common(vm, th);
     rb_native_mutex_unlock(&vm->gvl.lock);
 }
 
@@ -173,11 +218,11 @@ static void
 gvl_init(rb_vm_t *vm)
 {
     rb_native_mutex_initialize(&vm->gvl.lock);
-    rb_native_cond_initialize(&vm->gvl.cond);
     rb_native_cond_initialize(&vm->gvl.switch_cond);
     rb_native_cond_initialize(&vm->gvl.switch_wait_cond);
+    list_head_init(&vm->gvl.waitq);
     vm->gvl.acquired = 0;
-    vm->gvl.waiting = 0;
+    vm->gvl.timer = 0;
     vm->gvl.need_yield = 0;
     vm->gvl.wait_yield = 0;
 }
@@ -185,10 +230,16 @@ gvl_init(rb_vm_t *vm)
 static void
 gvl_destroy(rb_vm_t *vm)
 {
-    rb_native_cond_destroy(&vm->gvl.switch_wait_cond);
-    rb_native_cond_destroy(&vm->gvl.switch_cond);
-    rb_native_cond_destroy(&vm->gvl.cond);
-    rb_native_mutex_destroy(&vm->gvl.lock);
+    /*
+     * only called once at VM shutdown (not atfork), another thread
+     * may still grab vm->gvl.lock when calling gvl_release at
+     * the end of thread_start_func_2
+     */
+    if (0) {
+        rb_native_cond_destroy(&vm->gvl.switch_wait_cond);
+        rb_native_cond_destroy(&vm->gvl.switch_cond);
+        rb_native_mutex_destroy(&vm->gvl.lock);
+    }
     clear_thread_cache_altstack();
 }
 
@@ -433,7 +484,9 @@ native_thread_init(rb_thread_t *th)
 #ifdef USE_UBF_LIST
     list_node_init(&nd->ubf_list);
 #endif
-    rb_native_cond_initialize(&nd->sleep_cond);
+    rb_native_cond_initialize(&nd->cond.gvlq);
+    if (&nd->cond.gvlq != &nd->cond.intr)
+        rb_native_cond_initialize(&nd->cond.intr);
     ruby_thread_set_native(th);
 }
 
@@ -444,7 +497,11 @@ native_thread_init(rb_thread_t *th)
 static void
 native_thread_destroy(rb_thread_t *th)
 {
-    rb_native_cond_destroy(&th->native_thread_data.sleep_cond);
+    native_thread_data_t *nd = &th->native_thread_data;
+
+    rb_native_cond_destroy(&nd->cond.gvlq);
+    if (&nd->cond.gvlq != &nd->cond.intr)
+        rb_native_cond_destroy(&nd->cond.intr);
 
     /*
      * prevent false positive from ruby_thread_has_gvl_p if that
@@ -1012,17 +1069,6 @@ native_thread_create(rb_thread_t *th)
     return err;
 }
 
-#if (TIMER_IMPL & TIMER_THREAD_MASK)
-static void
-native_thread_join(pthread_t th)
-{
-    int err = pthread_join(th, 0);
-    if (err) {
-	rb_raise(rb_eThreadError, "native_thread_join() failed (%d)", err);
-    }
-}
-#endif /* TIMER_THREAD_MASK */
-
 #if USE_NATIVE_THREAD_PRIORITY
 
 static void
@@ -1064,15 +1110,15 @@ ubf_pthread_cond_signal(void *ptr)
 {
     rb_thread_t *th = (rb_thread_t *)ptr;
     thread_debug("ubf_pthread_cond_signal (%p)\n", (void *)th);
-    rb_native_cond_signal(&th->native_thread_data.sleep_cond);
+    rb_native_cond_signal(&th->native_thread_data.cond.intr);
 }
 
 static void
-native_sleep(rb_thread_t *th, struct timespec *timeout_rel)
+native_cond_sleep(rb_thread_t *th, struct timespec *timeout_rel)
 {
     struct timespec timeout;
     rb_nativethread_lock_t *lock = &th->interrupt_lock;
-    rb_nativethread_cond_t *cond = &th->native_thread_data.sleep_cond;
+    rb_nativethread_cond_t *cond = &th->native_thread_data.cond.intr;
 
     if (timeout_rel) {
 	/* Solaris cond_timedwait() return EINVAL if an argument is greater than
@@ -1164,17 +1210,30 @@ static void
 ubf_select(void *ptr)
 {
     rb_thread_t *th = (rb_thread_t *)ptr;
+    rb_vm_t *vm = th->vm;
+
     register_ubf_list(th);
 
     /*
      * ubf_wakeup_thread() doesn't guarantee to wake up a target thread.
      * Therefore, we repeatedly call ubf_wakeup_thread() until a target thread
-     * exit from ubf function.
-     * In the other hands, we shouldn't call rb_thread_wakeup_timer_thread()
-     * if running on timer thread because it may make endless wakeups.
+     * exit from ubf function.  We must designate a timer-thread to perform
+     * this operation.
      */
-    if (!pthread_equal(pthread_self(), timer_thread.id))
-	rb_thread_wakeup_timer_thread();
+   rb_native_mutex_lock(&vm->gvl.lock);
+   if (!vm->gvl.timer) {
+        native_thread_data_t *last;
+
+        last = list_tail(&vm->gvl.waitq, native_thread_data_t, ubf_list);
+        if (last) {
+            rb_native_cond_signal(&last->cond.gvlq);
+        }
+        else {
+            rb_thread_wakeup_timer_thread(0);
+        }
+    }
+    rb_native_mutex_unlock(&vm->gvl.lock);
+
     ubf_wakeup_thread(th);
 }
 
@@ -1211,39 +1270,16 @@ static int ubf_threads_empty(void) { return 1; }
 #define TT_DEBUG 0
 #define WRITE_CONST(fd, str) (void)(write((fd),(str),sizeof(str)-1)<0)
 
-/* 100ms.  10ms is too small for user level thread scheduling
- * on recent Linux (tested on 2.6.35)
- */
-#define TIME_QUANTUM_USEC (100 * 1000)
-
-#if TIMER_IMPL == TIMER_THREAD_SLEEPY
 static struct {
-    /*
-     * Read end of each pipe is closed inside timer thread for shutdown
-     * Write ends are closed by a normal Ruby thread during shutdown
-     */
+    /* pipes are closed in forked children when owner_process does not match */
     int normal[2];
-    int low[2];
 
     /* volatile for signal handler use: */
     volatile rb_pid_t owner_process;
 } timer_thread_pipe = {
     {-1, -1},
-    {-1, -1}, /* low priority */
 };
 
-NORETURN(static void async_bug_fd(const char *mesg, int errno_arg, int fd));
-static void
-async_bug_fd(const char *mesg, int errno_arg, int fd)
-{
-    char buff[64];
-    size_t n = strlcpy(buff, mesg, sizeof(buff));
-    if (n < sizeof(buff)-3) {
-	ruby_snprintf(buff+n, sizeof(buff)-n, "(%d)", fd);
-    }
-    rb_async_bug_errno(buff, errno_arg);
-}
-
 /* only use signal-safe system calls here */
 static void
 rb_thread_wakeup_timer_thread_fd(int fd)
@@ -1275,49 +1311,33 @@ rb_thread_wakeup_timer_thread_fd(int fd)
 }
 
 void
-rb_thread_wakeup_timer_thread(void)
+rb_thread_wakeup_timer_thread(int sig)
 {
     /* must be safe inside sighandler, so no mutex */
     if (timer_thread_pipe.owner_process == getpid()) {
-	rb_thread_wakeup_timer_thread_fd(timer_thread_pipe.normal[1]);
-    }
-}
-
-static void
-rb_thread_wakeup_timer_thread_low(void)
-{
-    if (timer_thread_pipe.owner_process == getpid()) {
-	rb_thread_wakeup_timer_thread_fd(timer_thread_pipe.low[1]);
-    }
-}
-
-/* VM-dependent API is not available for this function */
-static void
-consume_communication_pipe(int fd)
-{
-#define CCP_READ_BUFF_SIZE 1024
-    /* buffer can be shared because no one refers to them. */
-    static char buff[CCP_READ_BUFF_SIZE];
-    ssize_t result;
-
-    while (1) {
-	result = read(fd, buff, sizeof(buff));
-	if (result == 0) {
-	    return;
-	}
-	else if (result < 0) {
-	    int e = errno;
-	    switch (e) {
-	      case EINTR:
-		continue; /* retry */
-	      case EAGAIN:
-#if defined(EWOULDBLOCK) && EWOULDBLOCK != EAGAIN
-	      case EWOULDBLOCK:
-#endif
-		return;
-	      default:
-		async_bug_fd("consume_communication_pipe: read", e, fd);
-	    }
+        rb_thread_wakeup_timer_thread_fd(timer_thread_pipe.normal[1]);
+
+        /*
+         * system_working check is required because vm and main_thread are
+         * freed during shutdown
+         */
+        if (sig && system_working) {
+            volatile rb_execution_context_t *ec;
+            rb_vm_t *vm = GET_VM();
+            rb_thread_t *mth;
+
+            /*
+             * FIXME: root VM and main_thread should be static and not
+             * on heap for maximum safety (and startup/shutdown speed)
+             */
+            if (!vm) return;
+            mth = vm->main_thread;
+            if (!mth || !system_working) return;
+
+            /* this relies on GC for grace period before cont_free */
+            ec = ACCESS_ONCE(rb_execution_context_t *, mth->ec);
+
+            if (ec) RUBY_VM_SET_TRAP_INTERRUPT(ec);
 	}
     }
 }
@@ -1350,6 +1370,7 @@ set_nonblock(int fd)
 	rb_sys_fail(0);
 }
 
+/* communication pipe with timer thread and signal handler */
 static int
 setup_communication_pipe_internal(int pipes[2])
 {
@@ -1374,108 +1395,6 @@ setup_communication_pipe_internal(int pipes[2])
     return 0;
 }
 
-/* communication pipe with timer thread and signal handler */
-static int
-setup_communication_pipe(void)
-{
-    rb_pid_t owner = timer_thread_pipe.owner_process;
-
-    if (owner && owner != getpid()) {
-        CLOSE_INVALIDATE(normal[0]);
-        CLOSE_INVALIDATE(normal[1]);
-        CLOSE_INVALIDATE(low[0]);
-        CLOSE_INVALIDATE(low[1]);
-    }
-
-    if (setup_communication_pipe_internal(timer_thread_pipe.normal) < 0) {
-	return errno;
-    }
-    if (setup_communication_pipe_internal(timer_thread_pipe.low) < 0) {
-	return errno;
-    }
-
-    return 0;
-}
-
-/**
- * Let the timer thread sleep a while.
- *
- * The timer thread sleeps until woken up by rb_thread_wakeup_timer_thread() if only one Ruby thread is running.
- * @pre the calling context is in the timer thread.
- */
-static inline void
-timer_thread_sleep(rb_vm_t *vm)
-{
-    int result;
-    int need_polling;
-    struct pollfd pollfds[2];
-
-    pollfds[0].fd = timer_thread_pipe.normal[0];
-    pollfds[0].events = POLLIN;
-    pollfds[1].fd = timer_thread_pipe.low[0];
-    pollfds[1].events = POLLIN;
-
-    need_polling = !ubf_threads_empty();
-
-    if (SIGCHLD_LOSSY && !need_polling) {
-        rb_native_mutex_lock(&vm->waitpid_lock);
-        if (!list_empty(&vm->waiting_pids) || !list_empty(&vm->waiting_grps)) {
-            need_polling = 1;
-        }
-        rb_native_mutex_unlock(&vm->waitpid_lock);
-    }
-
-    if (vm->gvl.waiting > 0 || need_polling) {
-	/* polling (TIME_QUANTUM_USEC usec) */
-	result = poll(pollfds, 1, TIME_QUANTUM_USEC/1000);
-    }
-    else {
-	/* wait (infinite) */
-	result = poll(pollfds, numberof(pollfds), -1);
-    }
-
-    if (result == 0) {
-	/* maybe timeout */
-    }
-    else if (result > 0) {
-	consume_communication_pipe(timer_thread_pipe.normal[0]);
-	consume_communication_pipe(timer_thread_pipe.low[0]);
-    }
-    else { /* result < 0 */
-	int e = errno;
-	switch (e) {
-	  case EBADF:
-	  case EINVAL:
-	  case ENOMEM: /* from Linux man */
-	  case EFAULT: /* from FreeBSD man */
-	    rb_async_bug_errno("thread_timer: select", e);
-	  default:
-	    /* ignore */;
-	}
-    }
-}
-#endif /* TIMER_THREAD_SLEEPY */
-
-#if TIMER_IMPL == TIMER_THREAD_BUSY
-# define PER_NANO 1000000000
-void rb_thread_wakeup_timer_thread(void) {}
-static void rb_thread_wakeup_timer_thread_low(void) {}
-
-static rb_nativethread_lock_t timer_thread_lock;
-static rb_nativethread_cond_t timer_thread_cond;
-
-static inline void
-timer_thread_sleep(rb_vm_t *unused)
-{
-    struct timespec ts;
-    ts.tv_sec = 0;
-    ts.tv_nsec = TIME_QUANTUM_USEC * 1000;
-    ts = native_cond_timeout(&timer_thread_cond, ts);
-
-    native_cond_timedwait(&timer_thread_cond, &timer_thread_lock, &ts);
-}
-#endif /* TIMER_IMPL == TIMER_THREAD_BUSY */
-
 #if !defined(SET_CURRENT_THREAD_NAME) && defined(__linux__) && defined(PR_SET_NAME)
 # define SET_CURRENT_THREAD_NAME(name) prctl(PR_SET_NAME, name)
 #endif
@@ -1526,137 +1445,26 @@ native_set_another_thread_name(rb_nativethread_id_t thread_id, VALUE name)
     return name;
 }
 
-static void *
-thread_timer(void *p)
-{
-    rb_vm_t *vm = p;
-#ifdef HAVE_PTHREAD_SIGMASK /* mainly to enable SIGCHLD */
-    {
-        sigset_t mask;
-        sigemptyset(&mask);
-        pthread_sigmask(SIG_SETMASK, &mask, NULL);
-    }
-#endif
-
-    if (TT_DEBUG) WRITE_CONST(2, "start timer thread\n");
-
-#ifdef SET_CURRENT_THREAD_NAME
-    SET_CURRENT_THREAD_NAME("ruby-timer-thr");
-#endif
-
-#if TIMER_IMPL == TIMER_THREAD_BUSY
-    rb_native_mutex_initialize(&timer_thread_lock);
-    rb_native_cond_initialize(&timer_thread_cond);
-    rb_native_mutex_lock(&timer_thread_lock);
-#endif
-    while (system_working > 0) {
-
-	/* timer function */
-	ubf_wakeup_all_threads();
-	timer_thread_function(0);
-
-	if (TT_DEBUG) WRITE_CONST(2, "tick\n");
-
-        /* wait */
-	timer_thread_sleep(vm);
-    }
-#if TIMER_IMPL == TIMER_THREAD_BUSY
-    rb_native_mutex_unlock(&timer_thread_lock);
-    rb_native_cond_destroy(&timer_thread_cond);
-    rb_native_mutex_destroy(&timer_thread_lock);
-#endif
-
-    if (TT_DEBUG) WRITE_CONST(2, "finish timer thread\n");
-    return NULL;
-}
-
-#if (TIMER_IMPL & TIMER_THREAD_MASK)
 static void
 rb_thread_create_timer_thread(void)
 {
-    if (!timer_thread.created) {
-	size_t stack_size = 0;
-	int err;
-	pthread_attr_t attr;
-	rb_vm_t *vm = GET_VM();
+    /* we only create the pipe, and lazy-spawn */
+    rb_pid_t current = getpid();
+    rb_pid_t owner = timer_thread_pipe.owner_process;
 
-	err = pthread_attr_init(&attr);
-	if (err != 0) {
-	    rb_warn("pthread_attr_init failed for timer: %s, scheduling broken",
-		    strerror(err));
-	    return;
-        }
-# ifdef PTHREAD_STACK_MIN
-	{
-	    size_t stack_min = PTHREAD_STACK_MIN; /* may be dynamic, get only once */
-	    const size_t min_size = (4096 * 4);
-	    /* Allocate the machine stack for the timer thread
-	     * at least 16KB (4 pages).  FreeBSD 8.2 AMD64 causes
-	     * machine stack overflow only with PTHREAD_STACK_MIN.
-	     */
-	    enum {
-		needs_more_stack =
-#if defined HAVE_VALGRIND_MEMCHECK_H && defined __APPLE__
-		1
-#else
-		THREAD_DEBUG != 0
-#endif
-	    };
-	    stack_size = stack_min;
-	    if (stack_size < min_size) stack_size = min_size;
-	    if (needs_more_stack) {
-		stack_size += +((BUFSIZ - 1) / stack_min + 1) * stack_min;
-	    }
-	    err = pthread_attr_setstacksize(&attr, stack_size);
-	    if (err != 0) {
-		rb_bug("pthread_attr_setstacksize(.., %"PRIuSIZE") failed: %s",
-			stack_size, strerror(err));
-	    }
-	}
-# endif
+    if (owner && owner != current) {
+        CLOSE_INVALIDATE(normal[0]);
+        CLOSE_INVALIDATE(normal[1]);
+    }
 
-#if TIMER_IMPL == TIMER_THREAD_SLEEPY
-	err = setup_communication_pipe();
-	if (err) return;
-#endif /* TIMER_THREAD_SLEEPY */
+    if (setup_communication_pipe_internal(timer_thread_pipe.normal) < 0) return;
 
-	/* create timer thread */
-	if (timer_thread.created) {
-	    rb_bug("rb_thread_create_timer_thread: Timer thread was already created\n");
-	}
-	err = pthread_create(&timer_thread.id, &attr, thread_timer, vm);
-	pthread_attr_destroy(&attr);
-
-	if (err == EINVAL) {
-	    /*
-	     * Even if we are careful with our own stack use in thread_timer(),
-	     * any third-party libraries (eg libkqueue) which rely on __thread
-	     * storage can cause small stack sizes to fail.  So lets hope the
-	     * default stack size is enough for them:
-	     */
-	    stack_size = 0;
-	    err = pthread_create(&timer_thread.id, NULL, thread_timer, vm);
-	}
-	if (err != 0) {
-	    rb_warn("pthread_create failed for timer: %s, scheduling broken",
-		    strerror(err));
-	    if (stack_size) {
-		rb_warn("timer thread stack size: %"PRIuSIZE, stack_size);
-	    }
-	    else {
-		rb_warn("timer thread stack size: system default");
-	    }
-	    VM_ASSERT(err == 0);
-	    return;
-	}
-#if TIMER_IMPL == TIMER_THREAD_SLEEPY
-	/* validate pipe on this process */
-	timer_thread_pipe.owner_process = getpid();
-#endif /* TIMER_THREAD_SLEEPY */
-	timer_thread.created = 1;
+    if (owner != current) {
+        /* validate pipe on this process */
+        sigwait_th = THREAD_INVALID;
+        timer_thread_pipe.owner_process = current;
     }
 }
-#endif /* TIMER_IMPL & TIMER_THREAD_MASK */
 
 static int
 native_stop_timer_thread(void)
@@ -1665,24 +1473,6 @@ native_stop_timer_thread(void)
     stopped = --system_working <= 0;
 
     if (TT_DEBUG) fprintf(stderr, "stop timer thread\n");
-    if (stopped) {
-#if TIMER_IMPL == TIMER_THREAD_SLEEPY
-	/* kick timer thread out of sleep */
-	rb_thread_wakeup_timer_thread_fd(timer_thread_pipe.normal[1]);
-#endif
-
-	/* timer thread will stop looping when system_working <= 0: */
-	native_thread_join(timer_thread.id);
-
-	/*
-	 * don't care if timer_thread_pipe may fill up at this point.
-	 * If we restart timer thread, signals will be processed, if
-	 * we don't, it's because we're in a different child
-	 */
-
-	if (TT_DEBUG) fprintf(stderr, "joined timer thread\n");
-	timer_thread.created = 0;
-    }
     return stopped;
 }
 
@@ -1739,20 +1529,14 @@ ruby_stack_overflowed_p(const rb_thread_t *th, const void *addr)
 int
 rb_reserved_fd_p(int fd)
 {
-#if TIMER_IMPL == TIMER_THREAD_SLEEPY
     if ((fd == timer_thread_pipe.normal[0] ||
-	 fd == timer_thread_pipe.normal[1] ||
-	 fd == timer_thread_pipe.low[0] ||
-	 fd == timer_thread_pipe.low[1]) &&
+	 fd == timer_thread_pipe.normal[1]) &&
 	timer_thread_pipe.owner_process == getpid()) { /* async-signal-safe */
 	return 1;
     }
     else {
 	return 0;
     }
-#else
-    return 0;
-#endif
 }
 
 rb_nativethread_id_t
@@ -1803,7 +1587,7 @@ rb_sleep_cond_get(const rb_execution_context_t *ec)
 {
     rb_thread_t *th = rb_ec_thread_ptr(ec);
 
-    return &th->native_thread_data.sleep_cond;
+    return &th->native_thread_data.cond.intr;
 }
 
 void
@@ -1813,4 +1597,126 @@ rb_sleep_cond_put(rb_nativethread_cond_t *cond)
 }
 #endif /* USE_NATIVE_SLEEP_COND */
 
+int
+rb_sigwait_fd_get(const rb_thread_t *th)
+{
+    if (timer_thread_pipe.owner_process == getpid() &&
+        timer_thread_pipe.normal[0] >= 0) {
+        if (ATOMIC_PTR_CAS(sigwait_th, THREAD_INVALID, th) == THREAD_INVALID) {
+            return timer_thread_pipe.normal[0];
+        }
+    }
+    return -1; /* avoid thundering herd */
+}
+
+void
+rb_sigwait_fd_put(const rb_thread_t *th, int fd)
+{
+    const rb_thread_t *old;
+
+    VM_ASSERT(timer_thread_pipe.normal[0] == fd);
+    old = ATOMIC_PTR_EXCHANGE(sigwait_th, THREAD_INVALID);
+    if (old != th) assert(old == th);
+}
+
+#ifndef HAVE_PPOLL
+/* TODO: don't ignore sigmask */
+static int
+ruby_ppoll(struct pollfd *fds, nfds_t nfds,
+      const struct timespec *ts, const sigset_t *sigmask)
+{
+    int timeout_ms;
+
+    if (ts) {
+	int tmp, tmp2;
+
+	if (ts->tv_sec > INT_MAX/1000)
+	    timeout_ms = INT_MAX;
+	else {
+	    tmp = (int)(ts->tv_sec * 1000);
+	    /* round up 1ns to 1ms to avoid excessive wakeups for <1ms sleep */
+	    tmp2 = (int)((ts->tv_nsec + 999999L) / (1000L * 1000L));
+	    if (INT_MAX - tmp < tmp2)
+		timeout_ms = INT_MAX;
+	    else
+		timeout_ms = (int)(tmp + tmp2);
+	}
+    }
+    else
+	timeout_ms = -1;
+
+    return poll(fds, nfds, timeout_ms);
+}
+#  define ppoll(fds,nfds,ts,sigmask) ruby_ppoll((fds),(nfds),(ts),(sigmask))
+#endif
+
+void
+rb_sigwait_sleep(rb_thread_t *th, int sigwait_fd, const struct timespec *ts)
+{
+    struct pollfd pfd;
+
+    pfd.fd = sigwait_fd;
+    pfd.events = POLLIN;
+
+    if (!BUSY_WAIT_SIGNALS && ubf_threads_empty()) {
+        (void)ppoll(&pfd, 1, ts, 0);
+        check_signals_nogvl(th, sigwait_fd);
+    }
+    else {
+        struct timespec end, diff;
+        const struct timespec *to;
+        int n = 0;
+
+        if (ts) {
+            getclockofday(&end);
+            timespec_add(&end, ts);
+            diff = *ts;
+            ts = &diff;
+        }
+        /*
+         * tricky: this needs to return on spurious wakeup (no auto-retry).
+         * But we also need to distinguish between periodic quantum
+         * wakeups, so we care about the result of consume_communication_pipe
+         */
+        for (;;) {
+            to = sigwait_timeout(th, sigwait_fd, ts, &n);
+            if (n) return;
+            n = ppoll(&pfd, 1, to, 0);
+            if (check_signals_nogvl(th, sigwait_fd))
+                return;
+            if (n || (th && RUBY_VM_INTERRUPTED(th->ec)))
+                return;
+            if (ts && timespec_update_expire(&diff, &end))
+                return;
+        }
+    }
+}
+
+static void
+native_sleep(rb_thread_t *th, struct timespec *timeout_rel)
+{
+    int sigwait_fd = rb_sigwait_fd_get(th);
+
+    if (sigwait_fd >= 0) {
+        rb_native_mutex_lock(&th->interrupt_lock);
+        th->unblock.func = ubf_sigwait;
+        rb_native_mutex_unlock(&th->interrupt_lock);
+
+        GVL_UNLOCK_BEGIN(th);
+
+        if (!RUBY_VM_INTERRUPTED(th->ec)) {
+            rb_sigwait_sleep(th, sigwait_fd, timeout_rel);
+        }
+        else {
+            check_signals_nogvl(th, sigwait_fd);
+        }
+        unblock_function_clear(th);
+        GVL_UNLOCK_END(th);
+        rb_sigwait_fd_put(th, sigwait_fd);
+        rb_sigwait_fd_migrate(th->vm);
+    }
+    else {
+        native_cond_sleep(th, timeout_rel);
+    }
+}
 #endif /* THREAD_SYSTEM_DEPENDENT_IMPLEMENTATION */
diff --git a/thread_pthread.h b/thread_pthread.h
index 0566193eb5..60e0fe0ea3 100644
--- a/thread_pthread.h
+++ b/thread_pthread.h
@@ -22,7 +22,19 @@ typedef pthread_cond_t rb_nativethread_cond_t;
 
 typedef struct native_thread_data_struct {
     struct list_node ubf_list;
-    rb_nativethread_cond_t sleep_cond;
+#if defined(__GLIBC__) || defined(__FreeBSD__)
+    union
+#else
+    /*
+     * assume the platform condvars are badly implemented and have a
+     * "memory" of which mutex they're associated with
+     */
+    struct
+#endif
+    {
+        rb_nativethread_cond_t intr; /* th->interrupt_lock */
+        rb_nativethread_cond_t gvlq; /* vm->gvl.lock */
+    } cond;
 } native_thread_data_t;
 
 #undef except
@@ -32,12 +44,12 @@ typedef struct native_thread_data_struct {
 
 typedef struct rb_global_vm_lock_struct {
     /* fast path */
-    unsigned long acquired;
+    const struct rb_thread_struct *acquired;
     rb_nativethread_lock_t lock;
 
     /* slow path */
-    volatile unsigned long waiting;
-    rb_nativethread_cond_t cond;
+    struct list_head waitq;
+    const struct rb_thread_struct *timer;
 
     /* yield */
     rb_nativethread_cond_t switch_cond;
diff --git a/thread_win32.c b/thread_win32.c
index 2d5eac1ff4..6db1f25fa7 100644
--- a/thread_win32.c
+++ b/thread_win32.c
@@ -20,6 +20,8 @@
 
 #define native_thread_yield() Sleep(0)
 #define unregister_ubf_list(th)
+#define ubf_wakeup_all_threads() do {} while (0)
+#define ubf_threads_empty() (1)
 
 static volatile DWORD ruby_native_thread_key = TLS_OUT_OF_INDEXES;
 
@@ -680,18 +682,21 @@ static struct {
 static unsigned long __stdcall
 timer_thread_func(void *dummy)
 {
+    rb_vm_t *vm = GET_VM();
     thread_debug("timer_thread\n");
     rb_w32_set_thread_description(GetCurrentThread(), L"ruby-timer-thread");
     while (WaitForSingleObject(timer_thread.lock, TIME_QUANTUM_USEC/1000) ==
 	   WAIT_TIMEOUT) {
-	timer_thread_function(dummy);
+	timer_thread_function();
+	ruby_sigchld_handler(vm); /* probably no-op */
+	rb_threadptr_check_signal(vm->main_thread);
     }
     thread_debug("timer killed\n");
     return 0;
 }
 
 void
-rb_thread_wakeup_timer_thread(void)
+rb_thread_wakeup_timer_thread(int sig)
 {
     /* do nothing */
 }
@@ -768,6 +773,26 @@ rb_reserved_fd_p(int fd)
     return 0;
 }
 
+int
+rb_sigwait_fd_get(rb_thread_t *th)
+{
+    return -1; /* TODO */
+}
+
+NORETURN(void rb_sigwait_fd_put(rb_thread_t *, int));
+void
+rb_sigwait_fd_put(rb_thread_t *th, int fd)
+{
+    rb_bug("not implemented, should not be called");
+}
+
+NORETURN(void rb_sigwait_sleep(const rb_thread_t *, int, const struct timespec *));
+void
+rb_sigwait_sleep(const rb_thread_t *th, int fd, const struct timespec *ts)
+{
+    rb_bug("not implemented, should not be called");
+}
+
 rb_nativethread_id_t
 rb_nativethread_self(void)
 {
diff --git a/vm_core.h b/vm_core.h
index 01bb74ccde..e547e45f8b 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -564,10 +564,12 @@ typedef struct rb_vm_struct {
     VALUE self;
 
     rb_global_vm_lock_t gvl;
-    rb_nativethread_lock_t    thread_destruct_lock;
 
     struct rb_thread_struct *main_thread;
-    struct rb_thread_struct *running_thread;
+
+    /* persists across uncontended GVL release/acquire for time slice */
+    const struct rb_thread_struct *running_thread;
+
 #ifdef USE_SIGALTSTACK
     void *main_altstack;
 #endif
@@ -1581,7 +1583,7 @@ void rb_vm_pop_frame(rb_execution_context_t *ec);
 void rb_thread_start_timer_thread(void);
 void rb_thread_stop_timer_thread(void);
 void rb_thread_reset_timer_thread(void);
-void rb_thread_wakeup_timer_thread(void);
+void rb_thread_wakeup_timer_thread(int);
 
 static inline void
 rb_vm_living_threads_init(rb_vm_t *vm)
-- 
EW