Commit 59bed79f authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Boqun Feng
Browse files

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.

[Boqun: Add the subsystem tag in the title]

Reported-by: default avatarBreno Leitao <leitao@debian.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Reviewed-by: default avatarValentin Schneider <vschneid@redhat.com>
Reviewed-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/bd911cd9-1fe9-447c-85e0-ea811a1dc896@paulmck-laptop


Signed-off-by: default avatarBoqun Feng <boqun.feng@gmail.com>
parent 69381f38

kernel/context_tracking.c

+4 −5
Original line number Diff line number Diff line
@@ -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.

}



/*