[PATCH 4/4] workaround breakage from urcu v0.11.4

[Eric Wong <mwrap-perl@80x24.org>]

urcu v0.11.4+ introduced commit
7ca7fe9c03 (Make temporary variable in _rcu_dereference non-const, 2021-07-29)
which conflicts with our use of _LGPL_SOURCE.  In retrospect,
CMM_LOAD_SHARED and CMM_STORE_SHARED seem sufficient for our use
of the `totals' cds_lfht pointer since the constructur should always
fire before any threads are running.

This is fixed in urcu v0.12.4 and v0.13.2 (released 2022-08-18)
but I suspect older versions will live on in enterprise/LTS
distros for a long while.

Link: https://lore.kernel.org/lttng-dev/20220809181927.GA3718@dcvr/
---
 Mwrap.xs | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/Mwrap.xs b/Mwrap.xs
index 9e9f571..9a85f36 100644
--- a/Mwrap.xs
+++ b/Mwrap.xs
@@ -298,7 +298,7 @@ static struct src_loc *totals_add_rcu(struct src_loc *k)
 	struct cds_lfht *t;
 
 again:
-	t = rcu_dereference(totals);
+	t = CMM_LOAD_SHARED(totals);
 	if (!t) goto out_unlock;
 	cds_lfht_lookup(t, k->hval, loc_eq, k, &iter);
 	cur = cds_lfht_iter_get_node(&iter);
@@ -341,7 +341,7 @@ update_stats_rcu_lock(size_t *generation, size_t size, uintptr_t caller)
 	static const size_t xlen = sizeof(caller);
 	char *dst;
 	const COP *cop;
-	struct cds_lfht *t = rcu_dereference(totals);
+	struct cds_lfht *t = CMM_LOAD_SHARED(totals);
 
 	if (caa_unlikely(!t)) return 0;
 	if (locating++) goto out; /* do not recurse into another *alloc */
@@ -684,7 +684,7 @@ static void *dump_to_file(struct dump_arg *a)
 
 	++locating;
 	rcu_read_lock();
-	t = rcu_dereference(totals);
+	t = CMM_LOAD_SHARED(totals);
 	if (!t)
 		goto out_unlock;
 	cds_lfht_for_each_entry(t, &iter, l, hnode) {
@@ -805,8 +805,8 @@ BOOT:
 #ifndef PERL_IMPLICIT_CONTEXT
 	root_locating = &locating;
 #endif
-	totals = lfht_new();
-	if (!totals)
+	CMM_STORE_SHARED(totals, lfht_new());
+	if (!CMM_LOAD_SHARED(totals))
 		fprintf(stderr, "failed to allocate totals table\n");
 
 PROTOTYPES: ENABLE
@@ -851,7 +851,7 @@ PREINIT:
 CODE:
 	++locating;
 	rcu_read_lock();
-	t = rcu_dereference(totals);
+	t = CMM_LOAD_SHARED(totals);
 	if (t) {
 		bool err = false;
 
@@ -903,7 +903,7 @@ CODE:
 	uatomic_set(&total_bytes_dec, 0);
 
 	rcu_read_lock();
-	t = rcu_dereference(totals);
+	t = CMM_LOAD_SHARED(totals);
 	cds_lfht_for_each_entry(t, &iter, l, hnode) {
 		uatomic_set(&l->total, 0);
 		uatomic_set(&l->allocations, 0);
@@ -949,7 +949,7 @@ CODE:
 		XSRETURN_UNDEF;
 
 	rcu_read_lock();
-	t = rcu_dereference(totals);
+	t = CMM_LOAD_SHARED(totals);
 	if (!t) goto out_unlock;
 
 	cds_lfht_lookup(t, k->hval, loc_eq, k, &iter);