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23 October 2021, 00:29:24 UTC
Revision 0d4993563bde70bc7c7718686bc5c5c089733001 authored by Michael S. Tsirkin on 11 May 2010, 16:44:17 UTC, committed by Michael S. Tsirkin on 12 May 2010, 15:04:04 UTC 
vhost: fix barrier pairing
 
According to memory-barriers.txt, an smp memory barrier in guest
should always be paired with an smp memory barrier in host,
and I quote "a lack of appropriate pairing is almost certainly an
error". In case of vhost, failure to flush out used index
update before looking at the interrupt disable flag
could result in missed interrupts, resulting in
networking hang under stress.

This might happen when flags read bypasses used index write.
So we see interrupts disabled and do not interrupt, at the
same time guest writes flags value to enable interrupt,
reads an old used index value, thinks that
used ring is empty and waits for interrupt.

Note: the barrier we pair with here is in
drivers/virtio/virtio_ring.c, function
vring_enable_cb.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>
1 parent de02d72
History
Tip revision: 0d4993563bde70bc7c7718686bc5c5c089733001 authored by Michael S. Tsirkin on 11 May 2010, 16:44:17 UTC
vhost: fix barrier pairing
Tip revision: 0d49935
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