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

Similar to commits:

  51d7d5205d33 ("powerpc: Add smp_mb() to arch_spin_is_locked()")
  d86b8da04dfa ("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
problem.

Reported-and-tested-by: Davidlohr Bueso <dave@stgolabs.net>
Reported-by: Giovanni Gherdovich <ggherdovich@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org   # v4.2 and later
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
index 35a52a8..6bd0570 100644 (file)
--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -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.
+        */
+       smp_mb();
+       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() */
+       smp_mb();
        while (atomic_read(&lock->val) & _Q_LOCKED_MASK)
                cpu_relax();
 }