Commit 224fa355 authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

jump_label: Fix the fix, brown paper bags galore 



Per the example of:

  !atomic_cmpxchg(&key->enabled, 0, 1)

the inverse was written as:

  atomic_cmpxchg(&key->enabled, 1, 0)

except of course, that while !old is only true for old == 0, old is
true for everything except old == 0.

Fix it to read:

  atomic_cmpxchg(&key->enabled, 1, 0) == 1

such that only the 1->0 transition returns true and goes on to disable
the keys.

Fixes: 83ab38ef ("jump_label: Fix concurrency issues in static_key_slow_dec()")
Reported-by: default avatarDarrick J. Wong <djwong@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: default avatarDarrick J. Wong <djwong@kernel.org>
Link: https://lkml.kernel.org/r/20240731105557.GY33588@noisy.programming.kicks-ass.net
parent 6623b021

kernel/jump_label.c

+2 −2
Original line number Diff line number Diff line
@@ -236,7 +236,7 @@ void static_key_disable_cpuslocked(struct static_key *key) 
	}



	jump_label_lock();

	if (atomic_cmpxchg(&key->enabled, 1, 0))

	if (atomic_cmpxchg(&key->enabled, 1, 0) == 1)

		jump_label_update(key);

	jump_label_unlock();

}
@@ -289,7 +289,7 @@ static void __static_key_slow_dec_cpuslocked(struct static_key *key) 
		return;



	guard(mutex)(&jump_label_mutex);

	if (atomic_cmpxchg(&key->enabled, 1, 0))

	if (atomic_cmpxchg(&key->enabled, 1, 0) == 1)

		jump_label_update(key);

	else

		WARN_ON_ONCE(!static_key_slow_try_dec(key));