linux-kernel - [PATCH 5/6] rcu: Remove full memory barrier on RCU stall printout

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Date: Wed, 15 May 2024 14:53:31 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	Valentin Schneider <vschneid@...hat.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	Uladzislau Rezki <urezki@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	rcu <rcu@...r.kernel.org>
Subject: [PATCH 5/6] rcu: Remove full memory barrier on RCU stall printout

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@...nel.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