jump_label: Clarify condition in static_key_fast_inc_not_disabled()

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

commit 695ef79
Author: Thomas Gleixner <tglx@linutronix.de>
Date:   Mon, 10 Jun 2024 14:46:37 +0200

    jump_label: Clarify condition in static_key_fast_inc_not_disabled()

    The second part of

          if (v <= 0 || (v + 1) < 0)

    is not immediately obvious that it acts as overflow protection.

    Check explicitely for v == INT_MAX instead and add a proper comment how
    this is used at the call sites.

    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20240610124406.484973160@linutronix.de

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

Author: Waiman-Long

kernel/jump_label.c

@@ -132,12 +132,15 @@ bool static_key_fast_inc_not_disabled(struct static_key *key)
 	/*
 	 * Negative key->enabled has a special meaning: it sends
 	 * static_key_slow_inc/dec() down the slow path, and it is non-zero
-	 * so it counts as "enabled" in jump_label_update().  Note that
-	 * atomic_inc_unless_negative() checks >= 0, so roll our own.
+	 * so it counts as "enabled" in jump_label_update().
+	 *
+	 * The INT_MAX overflow condition is either used by the networking
+	 * code to reset or detected in the slow path of
+	 * static_key_slow_inc_cpuslocked().
 	 */
 	v = atomic_read(&key->enabled);
 	do {
-		if (v <= 0 || (v + 1) < 0)
+		if (v <= 0 || v == INT_MAX)
 			return false;
 	} while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v + 1)));