[4.2.y-ckt stable] Patch "locking, qspinlock: Fix spin_is_locked() and spin_unlock_wait()" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

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

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    https://git.launchpad.net/~canonical-kernel/linux/+git/linux-stable-ckt/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt12.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 5372e003c4ba75ab48e431ddc5103289f7793582 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz at infradead.org>
Date: Fri, 20 May 2016 18:04:36 +0200
Subject: locking,qspinlock: Fix spin_is_locked() and spin_unlock_wait()

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 at stgolabs.net>
Reported-by: Giovanni Gherdovich <ggherdovich at suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 include/asm-generic/qspinlock.h | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
index e2aadbc..7d633f1 100644
--- 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_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();
 }
--
2.7.4