[tip: locking/core] jump_label: Simplify and clarify static_key_fast_inc_cpus_locked()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     9bc2ff871f00437ad2f10c1eceff51aaa72b478f
Gitweb:        https://git.kernel.org/tip/9bc2ff871f00437ad2f10c1eceff51aaa72b478f
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Mon, 10 Jun 2024 14:46:39 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 11 Jun 2024 11:25:24 +02:00

jump_label: Simplify and clarify static_key_fast_inc_cpus_locked()

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

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20240610124406.548322963@xxxxxxxxxxxxx
---
 kernel/jump_label.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 4d06ec2..4ad5ed8 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -162,22 +162,24 @@ bool static_key_slow_inc_cpuslocked(struct static_key *key)
 	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;
 }