linux-kernel - [PATCH 13/20] x86/pvticketlock: make sure unlock is seen by everyone before checking waiters

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Date:	Wed,  3 Nov 2010 10:59:54 -0400
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Jan Beulich <JBeulich@...ell.com>, Avi Kivity <avi@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: [PATCH 13/20] x86/pvticketlock: make sure unlock is seen by everyone before checking waiters

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>

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
				block
	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@...rix.com>
---
 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 *lock)
 
 static __always_inline bool __ticket_lock_waiters(const struct arch_spinlock *lock)
 {
+	/*
+	 * 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;
 }
 #endif	/* CONFIG_PARAVIRT_SPINLOCKS */
-- 
1.7.2.3

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