linux-kernel - [PATCH] xen: disable interrupts before saving in percpu

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Date:	Tue, 03 Feb 2009 16:01:46 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Stable Kernel <stable@...nel.org>
Subject: [PATCH] xen: disable interrupts before saving in percpu

xen_mc_batch has a small preempt race where it takes the address of a
percpu variable immediately before disabling interrupts, thereby
leaving a small window in which we may migrate to another cpu and save
the flags in the wrong percpu variable.  Disable interrupts before
saving the old flags in a percpu.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
---
 arch/x86/xen/multicalls.h |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

===================================================================
--- a/arch/x86/xen/multicalls.h
+++ b/arch/x86/xen/multicalls.h
@@ -19,8 +19,10 @@
    paired with xen_mc_issue() */
 static inline void xen_mc_batch(void)
 {
+	unsigned long flags;
 	/* need to disable interrupts until this entry is complete */
-	local_irq_save(__get_cpu_var(xen_mc_irq_flags));
+	local_irq_save(flags);
+	__get_cpu_var(xen_mc_irq_flags) = flags;
 }
 
 static inline struct multicall_space xen_mc_entry(size_t args)


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