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Message-ID: <20240610124406.548322963@linutronix.de>
From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
 Sam Sun <samsun1006219@gmail.com>,
 x86@kernel.org,
 syzkaller-bugs@googlegroups.com,
 xrivendell7@gmail.com
Subject: [patch 4/4] jump_label: Simplify and clarify
 static_key_fast_inc_cpus_locked()
References: <87bk49xf15.ffs@tglx>
 <20240610124258.109097511@linutronix.de>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Date: Mon, 10 Jun 2024 14:46:39 +0200 (CEST)

Make the code more obvious and add proper comments to avoid future head
scratching.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/jump_label.c |   20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -162,22 +162,24 @@ bool static_key_slow_inc_cpuslocked(stru
 	if (static_key_fast_inc_not_disabled(key))
 		return true;
 
-	jump_label_lock();
-	if (atomic_read(&key->enabled) == 0) {
-		atomic_set(&key->enabled, -1);
+	guard(mutex)(&jump_label_mutex);
+	/* Try to mark it as 'enabling in progress. */
+	if (!atomic_cmpxchg(&key->enabled, 0, -1)) {
 		jump_label_update(key);
 		/*
-		 * Ensure that if the above cmpxchg loop observes our positive
-		 * value, it must also observe all the text changes.
+		 * Ensure that when static_key_fast_inc_not_disabled() or
+		 * static_key_slow_try_dec() observe the positive value,
+		 * they must also observe all the text changes.
 		 */
 		atomic_set_release(&key->enabled, 1);
 	} else {
-		if (WARN_ON_ONCE(!static_key_fast_inc_not_disabled(key))) {
-			jump_label_unlock();
+		/*
+		 * While holding the mutex this should never observe
+		 * anything else than a value >= 1 and succeed
+		 */
+		if (WARN_ON_ONCE(!static_key_fast_inc_not_disabled(key)))
 			return false;
-		}
 	}
-	jump_label_unlock();
 	return true;
 }