[Xen-devel] [PATCH] xen: disable interrupts before saving in percpu

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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@xxxxxxxxxx>
---
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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