On Thu, Feb 06, 2025 at 02:15:09AM -0800, Paul E. McKenney wrote:
> 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 <lei...@debian.org>
> Signed-off-by: Paul E. McKenney <paul...@kernel.org>
> Reviewed-by: Valentin Schneider <vschn...@redhat.com>
> Reviewed-by: Frederic Weisbecker <frede...@kernel.org>
>
Queued at:
git://git.kernel.org/pub/scm/linux/kernel/git/rcu/linux.git
misc.2025.02.06a
I added the "context_tracking:" tag to the title while I'm at it.
Regards,
Boqun
> 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.
> }
>
> /*
>
