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xen-devel
[Xen-devel] Re: [PATCH] Fix memory order issue inside pv spinlock
Jeremy Fitzhardinge wrote:
On 09/07/09 00:40, Yang, Xiaowei wrote:
barrier() can't prevent reads after it not being reordered with older
writes to different locates before it. Because of it, I can't bring up
4 HVM guests on one SMP machine. Use mb() instead.
Which read is happening too early? Is it "xl->spinners"? How does it fail?
Yes. If read of xl->spinners happens earlier than write 0 to xl->lock,
notifications to wake up other spinners can be omitted incorrectly, resulting in
others polling indefinitely (because of poll evtchn not pending) with the lock
is uncontended.
Thanks,
xiaowei
Thanks,
J
Signed-off-by: Yang Xiaowei <xiaowei.yang@xxxxxxxxx>
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 5601506..9dee5f8 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -324,7 +325,7 @@ static void xen_spin_unlock(struct raw_spinlock
*lock)
xl->lock = 0; /* release lock */
/* make sure unlock happens before kick */
- barrier();
+ mb();
if (unlikely(xl->spinners))
xen_spin_unlock_slow(xl);
Thanks,
Xiaowei
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