Patch "jump_label: Simplify and clarify static_key_fast_inc_cpus_locked()" has been added to the 6.6-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    jump_label: Simplify and clarify static_key_fast_inc_cpus_locked()

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     jump_label-simplify-and-clarify-static_key_fast_inc_.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 4182881be5a7e79ad85a9169acde3f0a0a03603e
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date:   Mon Jun 10 14:46:39 2024 +0200

    jump_label: Simplify and clarify static_key_fast_inc_cpus_locked()
    
    [ Upstream commit 9bc2ff871f00437ad2f10c1eceff51aaa72b478f ]
    
    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
    Stable-dep-of: 1d7f856c2ca4 ("jump_label: Fix static_key_slow_dec() yet again")
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 1ed269b2c4035..7374053bbe049 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -159,22 +159,24 @@ 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;
 }