linux-kernel - [PATCH 1/3] refscale: Fix misplaced data re-read

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Message-Id: <20231019115452.1215974-2-frederic@kernel.org>
Date:   Thu, 19 Oct 2023 13:54:50 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <neeraj.upadhyay@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Uladzislau Rezki <urezki@...il.com>, rcu <rcu@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: [PATCH 1/3] refscale: Fix misplaced data re-read

From: "Paul E. McKenney" <paulmck@...nel.org>

This commit fixes a misplaced data re-read in the typesafe code.
The reason that this was not noticed is that this is a performance test
with no writers, so a mismatch could not occur.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/rcu/refscale.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/refscale.c b/kernel/rcu/refscale.c
index 91a0fd0d4d9a..750a63e99539 100644
--- a/kernel/rcu/refscale.c
+++ b/kernel/rcu/refscale.c
@@ -655,12 +655,12 @@ static void typesafe_delay_section(const int nloops, const int udl, const int nd
 			goto retry;
 		}
 		un_delay(udl, ndl);
+		b = READ_ONCE(rtsp->a);
 		// Remember, seqlock read-side release can fail.
 		if (!rts_release(rtsp, start)) {
 			rcu_read_unlock();
 			goto retry;
 		}
-		b = READ_ONCE(rtsp->a);
 		WARN_ONCE(a != b, "Re-read of ->a changed from %u to %u.\n", a, b);
 		b = rtsp->b;
 		rcu_read_unlock();
-- 
2.34.1