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[Xen-devel] [PATCH 13/20] x86/pvticketlock: make sure unlock is seen by

To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Subject: [Xen-devel] [PATCH 13/20] x86/pvticketlock: make sure unlock is seen by everyone before checking waiters
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 3 Nov 2010 10:59:54 -0400
Cc: Nick Piggin <npiggin@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Linux Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>
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From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

If we don't put a real memory barrier between the unlocking increment
of the queue head and the check for lock waiters, we can end up with a
deadlock as a result of the unload write being reordered with respect
to the waiters read.  In effect, the check on the waiters count could
happen before the unlock, which leads to a deadlock:

        unlocker                locker
        check waiters (none)
                                check lock head (timeout)
                                add to waiters
        release lock            => deadlock

Adding a mb() barrier prevents the unlocker's waiter check from happening
before the lock release.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
 arch/x86/include/asm/spinlock.h |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 3deabca..b9a1aae 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -88,6 +88,13 @@ static inline void __ticket_sub_waiting(struct arch_spinlock 
 static __always_inline bool __ticket_lock_waiters(const struct arch_spinlock 
+       /*
+        * Make sure everyone sees the unlock write before we check the
+        * waiting count.  The processor ordering doesn't guarantee this
+        * because they're different memory locations.
+        */
+       mb();
        return ACCESS_ONCE(lock->waiting) != 0;

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