From: Frederic Weisbecker <frederic@kernel.org>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>,
Valentin Schneider <vschneid@redhat.com>,
"Paul E . McKenney" <paulmck@kernel.org>,
Boqun Feng <boqun.feng@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
Neeraj Upadhyay <neeraj.upadhyay@amd.com>,
Uladzislau Rezki <urezki@gmail.com>,
Zqiang <qiang.zhang1211@gmail.com>, rcu <rcu@vger.kernel.org>
Subject: [PATCH 5/6] rcu: Remove full memory barrier on RCU stall printout
Date: Wed, 15 May 2024 14:53:31 +0200 [thread overview]
Message-ID: <20240515125332.9306-6-frederic@kernel.org> (raw)
In-Reply-To: <20240515125332.9306-1-frederic@kernel.org>
RCU stall printout fetches the EQS state of a CPU with a preceding full
memory barrier. However there is nothing to order this read against at
this debugging stage. It is inherently racy when performed remotely.
Do a plain read instead.
This was the last user of rcu_dynticks_snap().
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/rcu/tree.c | 10 ----------
kernel/rcu/tree_stall.h | 4 ++--
2 files changed, 2 insertions(+), 12 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 02f6f3483482..04dde7473613 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -299,16 +299,6 @@ static void rcu_dynticks_eqs_online(void)
ct_state_inc(RCU_DYNTICKS_IDX);
}
-/*
- * Snapshot the ->dynticks counter with full ordering so as to allow
- * stable comparison of this counter with past and future snapshots.
- */
-static int rcu_dynticks_snap(int cpu)
-{
- smp_mb(); // Fundamental RCU ordering guarantee.
- return ct_dynticks_cpu_acquire(cpu);
-}
-
/*
* Return true if the snapshot returned from rcu_dynticks_snap()
* indicates that RCU is in an extended quiescent state.
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 460efecd077b..4b0e9d7c4c68 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -501,7 +501,7 @@ static void print_cpu_stall_info(int cpu)
}
delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
falsepositive = rcu_is_gp_kthread_starving(NULL) &&
- rcu_dynticks_in_eqs(rcu_dynticks_snap(cpu));
+ rcu_dynticks_in_eqs(ct_dynticks_cpu(cpu));
rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);
if (rcuc_starved)
// Print signed value, as negative values indicate a probable bug.
@@ -515,7 +515,7 @@ static void print_cpu_stall_info(int cpu)
rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
"!."[!delta],
ticks_value, ticks_title,
- rcu_dynticks_snap(cpu) & 0xffff,
+ ct_dynticks_cpu(cpu) & 0xffff,
ct_dynticks_nesting_cpu(cpu), ct_dynticks_nmi_nesting_cpu(cpu),
rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
--
2.44.0
next prev parent reply other threads:[~2024-05-15 12:53 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-15 12:53 [PATCH 0/6] rcu: Remove several redundant memory barriers Frederic Weisbecker
2024-05-15 12:53 ` [PATCH 1/6] rcu: Remove full ordering on second EQS snapshot Frederic Weisbecker
2024-05-15 17:32 ` Valentin Schneider
2024-05-15 12:53 ` [PATCH 2/6] rcu: Remove superfluous full memory barrier upon first " Frederic Weisbecker
2024-05-16 15:31 ` Valentin Schneider
2024-05-16 16:08 ` Frederic Weisbecker
2024-05-16 17:08 ` Valentin Schneider
2024-05-17 7:29 ` Andrea Parri
2024-05-17 11:40 ` Frederic Weisbecker
2024-05-17 16:27 ` Andrea Parri
2024-05-15 12:53 ` [PATCH 3/6] rcu/exp: " Frederic Weisbecker
2024-05-15 12:53 ` [PATCH 4/6] rcu: Remove full memory barrier on boot time eqs sanity check Frederic Weisbecker
2024-05-16 17:09 ` Valentin Schneider
2024-05-15 12:53 ` Frederic Weisbecker [this message]
2024-05-16 17:09 ` [PATCH 5/6] rcu: Remove full memory barrier on RCU stall printout Valentin Schneider
2024-05-15 12:53 ` [PATCH 6/6] rcu/exp: Remove redundant full memory barrier at the end of GP Frederic Weisbecker
2024-05-15 17:32 ` [PATCH 0/6] rcu: Remove several redundant memory barriers Valentin Schneider
2024-05-15 23:13 ` Frederic Weisbecker
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