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[Xen-devel] Re: [PATCH] Fix memory order issue inside pv spinlock

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: [Xen-devel] Re: [PATCH] Fix memory order issue inside pv spinlock
From: "Yang, Xiaowei" <xiaowei.yang@xxxxxxxxx>
Date: Thu, 10 Sep 2009 00:30:00 +0800
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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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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