locking/static_key: Fix false positive warnings on concurrent dec/inc

Even though the atomic_dec_and_mutex_lock() in
__static_key_slow_dec_cpuslocked() can never see a negative value in
key->enabled the subsequent sanity check is re-reading key->enabled, which may
have been set to -1 in the meantime by static_key_slow_inc_cpuslocked().

                CPU  A                               CPU B

 __static_key_slow_dec_cpuslocked():          static_key_slow_inc_cpuslocked():
                               # enabled = 1
   atomic_dec_and_mutex_lock()
                               # enabled = 0
                                              atomic_read() == 0
                                              atomic_set(-1)
                               # enabled = -1
   val = atomic_read()
   # Oops - val == -1!

The test case is TCP's clean_acked_data_enable() / clean_acked_data_disable()
as tickled by KTLS (net/ktls).
Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Tested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: ard.biesheuvel@linaro.org
Cc: oss-drivers@netronome.com
Cc: pbonzini@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/jump_label.c

@@ -206,6 +206,8 @@ static void __static_key_slow_dec_cpuslocked(struct static_key *key,
 					   unsigned long rate_limit,
 					   struct delayed_work *work)
 {
+	int val;
+
 	lockdep_assert_cpus_held();
 
 	/*
@@ -215,18 +217,21 @@ static void __static_key_slow_dec_cpuslocked(struct static_key *key,
 	 * returns is unbalanced, because all other static_key_slow_inc()
 	 * instances block while the update is in progress.
 	 */
-	if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex)) {
-		WARN(atomic_read(&key->enabled) < 0,
-		     "jump label: negative count!\n");
+	val = atomic_fetch_add_unless(&key->enabled, -1, 1);
+	if (val != 1) {
+		WARN(val < 0, "jump label: negative count!\n");
 		return;
 	}
 
+	jump_label_lock();
+	if (atomic_dec_and_test(&key->enabled)) {
 		if (rate_limit) {
 			atomic_inc(&key->enabled);
 			schedule_delayed_work(work, rate_limit);
 		} else {
 			jump_label_update(key);
 		}
+	}
 	jump_label_unlock();
 }