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context_tracking: Make RCU watch ct_kernel_exit_state() warning

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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.

[Boqun: Add the subsystem tag in the title]

Reported-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/bd911cd9-1fe9-447c-85e0-ea811a1dc896@paulmck-laptop
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
This commit is contained in:
Paul E. McKenney 2025-02-06 02:15:09 -08:00 committed by Boqun Feng
parent 69381f3828
commit 59bed79ffd
1 changed files with 4 additions and 5 deletions
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9
kernel/context_tracking.c
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@ -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.
}
/*