Commit 54cf809b authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Linus Torvalds
Browse files

locking,qspinlock: Fix spin_is_locked() and spin_unlock_wait()

Similar to commits:

  51d7d520 ("powerpc: Add smp_mb() to arch_spin_is_locked()")

 ("arm64: spinlock: serialise spin_unlock_wait against concurrent lockers")

qspinlock suffers from the fact that the _Q_LOCKED_VAL store is
unordered inside the ACQUIRE of the lock.

And while this is not a problem for the regular mutual exclusive
critical section usage of spinlocks, it breaks creative locking like:

	spin_lock(A)			spin_lock(B)
	spin_unlock_wait(B)		if (!spin_is_locked(A))
	do_something()			  do_something()

In that both CPUs can end up running do_something at the same time,
because our _Q_LOCKED_VAL store can drop past the spin_unlock_wait()
spin_is_locked() loads (even on x86!!).

To avoid making the normal case slower, add smp_mb()s to the less used
spin_unlock_wait() / spin_is_locked() side of things to avoid this
Reported-and-tested-by: default avatarDavidlohr Bueso <>
Reported-by: default avatarGiovanni Gherdovich <>
Signed-off-by: default avatarPeter Zijlstra (Intel) <>
Cc:   # v4.2 and later
Signed-off-by: default avatarLinus Torvalds <>
parent b99a9e87
......@@ -28,7 +28,30 @@
static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
return atomic_read(&lock->val);
* queued_spin_lock_slowpath() can ACQUIRE the lock before
* issuing the unordered store that sets _Q_LOCKED_VAL.
* See both smp_cond_acquire() sites for more detail.
* This however means that in code like:
* spin_lock(A) spin_lock(B)
* spin_unlock_wait(B) spin_is_locked(A)
* do_something() do_something()
* Both CPUs can end up running do_something() because the store
* setting _Q_LOCKED_VAL will pass through the loads in
* spin_unlock_wait() and/or spin_is_locked().
* Avoid this by issuing a full memory barrier between the spin_lock()
* and the loads in spin_unlock_wait() and spin_is_locked().
* Note that regular mutual exclusion doesn't care about this
* delayed store.
return atomic_read(&lock->val) & _Q_LOCKED_MASK;
......@@ -108,6 +131,8 @@ static __always_inline void queued_spin_unlock(struct qspinlock *lock)
static inline void queued_spin_unlock_wait(struct qspinlock *lock)
/* See queued_spin_is_locked() */
while (atomic_read(&lock->val) & _Q_LOCKED_MASK)
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