linux-kernel - [PATCH RFC context_tracking] Make RCU watch ct_kernel_exit

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Message-ID: <19bebbeb-db36-4801-bc3d-2c39989b6152@paulmck-laptop>
Date: Sat, 1 Feb 2025 10:44:02 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: linux-kernel@...r.kernel.org
Cc: frederic@...nel.org, vschneid@...hat.com, leitao@...ian.org
Subject: [PATCH RFC context_tracking] Make RCU watch ct_kernel_exit_state()
 warning

The WARN_ON_ONCE() in ct_kernel_exit_state() follows the call to
ct_state_inc(), which means that RCU is not watching this WARN_ON_ONCE().
This can (and does) result in extraneous lockdep warnings when this
WARN_ON_ONCE() triggers.  These extraneous warnings are the opposite
of helpful.

Therefore, invert the WARN_ON_ONCE() condition and move it before the
call to ct_state_inc().  This does mean that the ct_state_inc() return
value can no longer be used in the WARN_ON_ONCE() condition, so discard
this return value and instead use a call to rcu_is_watching_curr_cpu().
This call is executed only in CONFIG_RCU_EQS_DEBUG=y kernels, so there
is no added overhead in production use.

Reported-by: Breno Leitao <leitao@...ian.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>
Cc: Valentin Schneider <vschneid@...hat.com>

diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 938c48952d26..fb5be6e9b423 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -80,17 +80,16 @@ static __always_inline void rcu_task_trace_heavyweight_exit(void)
  */
 static noinstr void ct_kernel_exit_state(int offset)
 {
-	int seq;
-
 	/*
 	 * CPUs seeing atomic_add_return() must see prior RCU read-side
 	 * critical sections, and we also must force ordering with the
 	 * next idle sojourn.
 	 */
 	rcu_task_trace_heavyweight_enter();  // Before CT state update!
-	seq = ct_state_inc(offset);
-	// RCU is no longer watching.  Better be in extended quiescent state!
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & CT_RCU_WATCHING));
+	// RCU is still watching.  Better not be in extended quiescent state!
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !rcu_is_watching_curr_cpu());
+	(void)ct_state_inc(offset);
+	// RCU is no longer watching.
 }

 /*