Merge tag 'locking-core-2022-12-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull locking updates from Ingo Molnar:
 "Two changes in this cycle:

   - a micro-optimization in static_key_slow_inc_cpuslocked()

   - fix futex death-notification wakeup bug"

* tag 'locking-core-2022-12-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  futex: Resend potentially swallowed owner death notification
  jump_label: Use atomic_try_cmpxchg() in static_key_slow_inc_cpuslocked()

Author: torvalds

kernel/futex/core.c

@@ -638,6 +638,7 @@ static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
 			      bool pi, bool pending_op)
 {
 	u32 uval, nval, mval;
+	pid_t owner;
 	int err;
 
 	/* Futex address must be 32bit aligned */
@@ -659,6 +660,10 @@ static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
 	 * 2. A woken up waiter is killed before it can acquire the
 	 *    futex in user space.
 	 *
+	 * In the second case, the wake up notification could be generated
+	 * by the unlock path in user space after setting the futex value
+	 * to zero or by the kernel after setting the OWNER_DIED bit below.
+	 *
 	 * In both cases the TID validation below prevents a wakeup of
 	 * potential waiters which can cause these waiters to block
 	 * forever.
@@ -667,24 +672,27 @@ static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
 	 *
 	 *	1) task->robust_list->list_op_pending != NULL
 	 *	   @pending_op == true
-	 *	2) User space futex value == 0
+	 *	2) The owner part of user space futex value == 0
 	 *	3) Regular futex: @pi == false
 	 *
 	 * If these conditions are met, it is safe to attempt waking up a
 	 * potential waiter without touching the user space futex value and
-	 * trying to set the OWNER_DIED bit. The user space futex value is
-	 * uncontended and the rest of the user space mutex state is
-	 * consistent, so a woken waiter will just take over the
-	 * uncontended futex. Setting the OWNER_DIED bit would create
-	 * inconsistent state and malfunction of the user space owner died
-	 * handling.
+	 * trying to set the OWNER_DIED bit. If the futex value is zero,
+	 * the rest of the user space mutex state is consistent, so a woken
+	 * waiter will just take over the uncontended futex. Setting the
+	 * OWNER_DIED bit would create inconsistent state and malfunction
+	 * of the user space owner died handling. Otherwise, the OWNER_DIED
+	 * bit is already set, and the woken waiter is expected to deal with
+	 * this.
 	 */
-	if (pending_op && !pi && !uval) {
+	owner = uval & FUTEX_TID_MASK;
+
+	if (pending_op && !pi && !owner) {
 		futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
 		return 0;
 	}
 
-	if ((uval & FUTEX_TID_MASK) != task_pid_vnr(curr))
+	if (owner != task_pid_vnr(curr))
 		return 0;
 
 	/*

kernel/jump_label.c

@@ -115,8 +115,6 @@ EXPORT_SYMBOL_GPL(static_key_count);
 
 void static_key_slow_inc_cpuslocked(struct static_key *key)
 {
-	int v, v1;
-
 	STATIC_KEY_CHECK_USE(key);
 	lockdep_assert_cpus_held();
 
@@ -132,11 +130,9 @@ void static_key_slow_inc_cpuslocked(struct static_key *key)
 	 * so it counts as "enabled" in jump_label_update().  Note that
 	 * atomic_inc_unless_negative() checks >= 0, so roll our own.
 	 */
-	for (v = atomic_read(&key->enabled); v > 0; v = v1) {
-		v1 = atomic_cmpxchg(&key->enabled, v, v + 1);
-		if (likely(v1 == v))
+	for (int v = atomic_read(&key->enabled); v > 0; )
+		if (likely(atomic_try_cmpxchg(&key->enabled, &v, v + 1)))
 			return;
-	}
 
 	jump_label_lock();
 	if (atomic_read(&key->enabled) == 0) {