[PATCH 4.4 71/86] locking,qspinlock: Fix spin_is_locked() and spin_unlock_wait()

Greg Kroah-Hartman
2016-05-30T20:50:41Z

From: Peter Zijlstra <peterz@infradead.org>

commit 54cf809b9512be95f53ed4a5e3b631d1ac42f0fa upstream.

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
include/asm-generic/qspinlock.h | 27 ++++++++++++++++++++++++++-
1 file changed, 26 insertions(+), 1 deletion(-)

--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -27,7 +27,30 @@
*/
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;
}

/**
@@ -107,6 +130,8 @@ static __always_inline void queued_spin_
*/
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();
}