Re: [PATCH v2 1/6] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW_VAL

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 14, 2015 at 10:13:32PM -0400, Waiman Long wrote:
> The smp_store_release() is not a full barrier. In order to avoid missed
> wakeup, we may need to add memory barrier around locked and cpu state
> variables adding to complexity. As the chance of spurious wakeup is very
> low, it is easier and safer to just do an unconditional kick at unlock
> time.

I have the below patch. We need that rmb in there anyhow for the hash to
work.

---
Subject: locking/pvqspinlock: Order pv_unhash after cmpxchg on unlock slowpath
From: Will Deacon <will.deacon@arm.com>
Date: Mon, 13 Jul 2015 16:58:30 +0100

When we unlock in __pv_queued_spin_unlock, a failed cmpxchg on the lock
value indicates that we need to take the slow-path and unhash the
corresponding node blocked on the lock.

Since a failed cmpxchg does not provide any memory-ordering guarantees,
it is possible that the node data could be read before the cmpxchg on
weakly-ordered architectures and therefore return a stale value, leading
to hash corruption and/or a BUG().

This patch adds an smb_rmb() following the failed cmpxchg operation, so
that the unhashing is ordered after the lock has been checked.

Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: Steve Capper <Steve.Capper@arm.com>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
[peterz: More comments]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20150713155830.GL2632@arm.com
---
 kernel/locking/qspinlock_paravirt.h |   23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -244,13 +244,17 @@ static void pv_wait_head(struct qspinloc
 		if (!lp) { /* ONCE */
 			lp = pv_hash(lock, pn);
 			/*
-			 * lp must be set before setting _Q_SLOW_VAL
+			 * We must hash before setting _Q_SLOW_VAL, such that
+			 * when we observe _Q_SLOW_VAL in __pv_queued_spin_unlock()
+			 * we'll be sure to be able to observe our hash entry.
 			 *
-			 * [S] lp = lock                [RmW] l = l->locked = 0
-			 *     MB                             MB
-			 * [S] l->locked = _Q_SLOW_VAL  [L]   lp
+			 *   [S] pn->state
+			 *   [S] <hash>                 [Rmw] l->locked == _Q_SLOW_VAL
+			 *       MB                           RMB
+			 * [RmW] l->locked = _Q_SLOW_VAL  [L] <unhash>
+			 *                                [L] pn->state
 			 *
-			 * Matches the cmpxchg() in __pv_queued_spin_unlock().
+			 * Matches the smp_rmb() in __pv_queued_spin_unlock().
 			 */
 			if (!cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL)) {
 				/*
@@ -306,6 +310,15 @@ __visible void __pv_queued_spin_unlock(s
 	}
 
 	/*
+	 * A failed cmpxchg doesn't provide any memory-ordering guarantees,
+	 * so we need a barrier to order the read of the node data in
+	 * pv_unhash *after* we've read the lock being _Q_SLOW_VAL.
+	 *
+	 * Matches the cmpxchg() in pv_wait_head() setting _Q_SLOW_VAL.
+	 */
+	smp_rmb();
+
+	/*
 	 * Since the above failed to release, this must be the SLOW path.
 	 * Therefore start by looking up the blocked node and unhashing it.
 	 */