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xen: use stronger barrier after unlocking lock 

We need to have a stronger barrier between releasing the lock and
checking for any waiting spinners.  A compiler barrier is not sufficient
because the CPU's ordering rules do not prevent the read xl->spinners
from happening before the unlock assignment, as they are different
memory locations.

We need to have an explicit barrier to enforce the write-read ordering
to different memory locations.

Because of it, I can't bring up > 4 HVM guests on one SMP machine.

[ Code and commit comments expanded -J ]

[ Impact: avoid deadlock when using Xen PV spinlocks ]

Signed-off-by: Yang Xiaowei <xiaowei.yang@intel.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
This commit is contained in:
Yang Xiaowei 2009-09-09 12:44:52 -07:00 committed by Jeremy Fitzhardinge
parent 4d576b57b5
commit 2496afbf1e
1 changed files with 7 additions and 2 deletions
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9
arch/x86/xen/spinlock.c
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@ -326,8 +326,13 @@ static void xen_spin_unlock(struct raw_spinlock *lock)
smp_wmb(); /* make sure no writes get moved after unlock */
xl->lock = 0; /* release lock */
/* make sure unlock happens before kick */
barrier();
/*
* Make sure unlock happens before checking for waiting
* spinners. We need a strong barrier to enforce the
* write-read ordering to different memory locations, as the
* CPU makes no implied guarantees about their ordering.
*/
mb();
if (unlikely(xl->spinners))
xen_spin_unlock_slow(xl);