linux-kernel - [PATCH 15/20] x86/ticketlock: prevent compiler reordering into locked region

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Message-Id: <12a3251ccc2f0265678e3d1a3ad2faf6fb7f4725.1288794124.git.jeremy.fitzhardinge@citrix.com>
Date:	Wed,  3 Nov 2010 10:59:56 -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 15/20] x86/ticketlock: prevent compiler reordering into locked region

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

Add a barrier() at the end of __raw_spin_unlock() to prevent instructions
from after the locked region from being reordered into it.  In theory doing
so shouldn't cause any problems, but in practice, the system locks up
under load...

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
---
 arch/x86/include/asm/spinlock.h |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index d6de5e7..158b330 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -189,6 +189,8 @@ static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 	next = lock->tickets.head + 1;
 	__ticket_unlock_release(lock);
 	__ticket_unlock_kick(lock, next);
+
+	barrier();		/* prevent reordering into locked region */
 }
 
 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
-- 
1.7.2.3

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