From: "Paul E. McKenney" paulmck@kernel.org
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-lapto... Signed-off-by: Boqun Feng boqun.feng@gmail.com --- kernel/context_tracking.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-)
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. }
/*