linux-kernel - [PATCH 2/2] rcu: do not disable gp stall detection in rcu_cpu_stall

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Message-Id: <20210521155624.174524-2-senozhatsky@chromium.org>
Date:   Sat, 22 May 2021 00:56:24 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: [PATCH 2/2] rcu: do not disable gp stall detection in rcu_cpu_stall_reset()

rcu_cpu_stall_reset() is one of the functions virtual CPUs
execute during VM resume in order to handle jiffies skew
that can trigger false positive stall warnings. Paul has
pointed out that this approach is problematic because
rcu_cpu_stall_reset() disables RCU grace period stall-detection
virtually forever, while in fact it can just restart the
stall-detection timeout.

Suggested-by: "Paul E. McKenney" <paulmck@...nel.org>
Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
---
 kernel/rcu/tree_stall.h | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index bc689911a81d..b9b52f91e5b8 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -119,17 +119,14 @@ static void panic_on_rcu_stall(void)
 }
 
 /**
- * rcu_cpu_stall_reset - prevent further stall warnings in current grace period
- *
- * Set the stall-warning timeout way off into the future, thus preventing
- * any RCU CPU stall-warning messages from appearing in the current set of
- * RCU grace periods.
+ * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
  *
  * The caller must disable hard irqs.
  */
 void rcu_cpu_stall_reset(void)
 {
-	WRITE_ONCE(rcu_state.jiffies_stall, jiffies + ULONG_MAX / 2);
+	WRITE_ONCE(rcu_state.jiffies_stall,
+		   jiffies + rcu_jiffies_till_stall_check());
 }
 
 //////////////////////////////////////////////////////////////////////////////
-- 
2.31.1.818.g46aad6cb9e-goog