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[Xen-devel] [RFC PATCH] dpci: Put the dpci back on the list if running on another CPU.

There is race when we clear the STATE_SCHED in the softirq
- which allows the 'raise_softirq_for' to progress and
schedule another dpci. During that time the other CPU could
receive an interrupt and calls 'raise_softirq_for' and put
the dpci on its per-cpu list. There would be two 'dpci_softirq'
running at the same time (on different CPUs) where the
dpci state is STATE_RUN (and STATE_SCHED is cleared). This
ends up hitting:

 if ( test_and_set_bit(STATE_RUN, &pirq_dpci->state) )

Instead of that put the dpci back on the per-cpu list to deal
with later.

The reason we can get his with this is when an interrupt
affinity is set over multiple CPUs.

Another potential fix would be to add a guard in the raise_softirq_for
to check for 'STATE_RUN' bit being set and not schedule the
dpci until that bit has been cleared.

Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Reported-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
 xen/drivers/passthrough/io.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/xen/drivers/passthrough/io.c b/xen/drivers/passthrough/io.c
index ae050df..9b77334 100644
--- a/xen/drivers/passthrough/io.c
+++ b/xen/drivers/passthrough/io.c
@@ -804,7 +804,17 @@ static void dpci_softirq(void)
         d = pirq_dpci->dom;
         smp_mb(); /* 'd' MUST be saved before we set/clear the bits. */
         if ( test_and_set_bit(STATE_RUN, &pirq_dpci->state) )
-            BUG();
+        {
+            unsigned long flags;
+            /* Put back on the list and retry. */
+            local_irq_save(flags);
+            list_add_tail(&pirq_dpci->softirq_list, &this_cpu(dpci_list));
+            local_irq_restore(flags);
+            raise_softirq(HVM_DPCI_SOFTIRQ);
+            continue;
+        }
          * The one who clears STATE_SCHED MUST refcount the domain.

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