jump_label: Fix static_key_slow_dec() yet again

While commit 83ab38ef0a0b ("jump_label: Fix concurrency issues in
static_key_slow_dec()") fixed one problem, it created yet another,
notably the following is now possible:

slow_dec
if (try_dec) // dec_not_one-ish, false
// enabled == 1
slow_inc
if (inc_not_disabled) // inc_not_zero-ish
// enabled == 2
return

guard((mutex)(&jump_label_mutex);
if (atomic_cmpxchg(1,0)==1) // false, we're 2

slow_dec
if (try-dec) // dec_not_one, true
// enabled == 1
return
else
try_dec() // dec_not_one, false
WARN

Use dec_and_test instead of cmpxchg(), like it was prior to
83ab38ef0a0b. Add a few WARNs for the paranoid.

Fixes: 83ab38ef0a0b ("jump_label: Fix concurrency issues in static_key_slow_dec()")
Reported-by: "Darrick J. Wong" <[e-mail]>
Tested-by: Klara Modin <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>

kernel/jump_label.c

···
 		jump_label_update(key);
 		/*
 		 * Ensure that when static_key_fast_inc_not_disabled() or
-		 * static_key_slow_try_dec() observe the positive value,
+		 * static_key_dec_not_one() observe the positive value,
 		 * they must also observe all the text changes.
 		 */
 		atomic_set_release(&key->enabled, 1);
···
 }
 EXPORT_SYMBOL_GPL(static_key_disable);
 
-static bool static_key_slow_try_dec(struct static_key *key)
+static bool static_key_dec_not_one(struct static_key *key)
 {
 	int v;
 
···
 		 * enabled. This suggests an ordering problem on the user side.
 		 */
 		WARN_ON_ONCE(v < 0);
+
+		/*
+		 * Warn about underflow, and lie about success in an attempt to
+		 * not make things worse.
+		 */
+		if (WARN_ON_ONCE(v == 0))
+			return true;
+
 		if (v <= 1)
 			return false;
 	} while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v - 1)));
···
 static void __static_key_slow_dec_cpuslocked(struct static_key *key)
 {
 	lockdep_assert_cpus_held();
+	int val;
 
-	if (static_key_slow_try_dec(key))
+	if (static_key_dec_not_one(key))
 		return;
 
 	guard(mutex)(&jump_label_mutex);
-	if (atomic_cmpxchg(&key->enabled, 1, 0) == 1)
+	val = atomic_read(&key->enabled);
+	/*
+	 * It should be impossible to observe -1 with jump_label_mutex held,
+	 * see static_key_slow_inc_cpuslocked().
+	 */
+	if (WARN_ON_ONCE(val == -1))
+		return;
+	/*
+	 * Cannot already be 0, something went sideways.
+	 */
+	if (WARN_ON_ONCE(val == 0))
+		return;
+
+	if (atomic_dec_and_test(&key->enabled))
 		jump_label_update(key);
-	else
-		WARN_ON_ONCE(!static_key_slow_try_dec(key));
 }
 
 static void __static_key_slow_dec(struct static_key *key)
···
 {
 	STATIC_KEY_CHECK_USE(key);
 
-	if (static_key_slow_try_dec(key))
+	if (static_key_dec_not_one(key))
 		return;
 
 	schedule_delayed_work(work, timeout);