Commit 54cf809b authored May 20, 2016 by Peter Zijlstra Committed by Linus Torvalds May 20, 2016

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



Similar to commits:

  51d7d520 ("powerpc: Add smp_mb() to arch_spin_is_locked()")
  d86b8da0 ("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>

parent b99a9e87

include/asm-generic/qspinlock.h

+26 −1

Original line number	Diff line number	Diff line
		@@ -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();
		}