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@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20240610124406.548322963@linutronix.de
Stable-dep-of: 1d7f856c2ca4 ("jump_label: Fix static_key_slow_dec() yet again")
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index c6ac0d0377d72601f70643e211e18a7eace8da9b..781a1298ce2c2a02b5cdb9d83a85f181ebf21c92 100644 (file)
--- 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;
 }