jump_label: Fix the fix, brown paper bags galore

JIRA: https://issues.redhat.com/browse/RHEL-68940

commit 224fa35
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Wed, 31 Jul 2024 12:43:21 +0200

    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: 83ab38e ("jump_label: Fix concurrency issues in static_key_slow_dec()")
    Reported-by: Darrick J. Wong <djwong@kernel.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Tested-by: Darrick J. Wong <djwong@kernel.org>
    Link: https://lkml.kernel.org/r/20240731105557.GY33588@noisy.programming.kicks-ass.net

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

kernel/jump_label.c

@@ -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));