Re: [PATCH 2/2] virtio_net: fix race in RX VQ processing
From: Sergei Shtylyov <hidden>
Date: 2013-07-08 12:52:31
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Hello. On 08-07-2013 13:04, Michael S. Tsirkin wrote:
virtio net called virtqueue_enable_cq on RX path after napi_complete, so with NAPI_STATE_SCHED clear - outside the implicit napi lock. This violates the requirement to synchronize virtqueue_enable_cq wrt virtqueue_add_buf. In particular, used event can move backwards, causing us to lose interrupts. In a debug build, this can trigger panic within START_USE.
Jason Wang reports that he can trigger the races artificially, by adding udelay() in virtqueue_enable_cb() after virtio_mb().
However, we must call napi_complete to clear NAPI_STATE_SCHED before polling the virtqueue for used buffers, otherwise napi_schedule_prep in a callback will fail, causing us to lose RX events.
To fix, call virtqueue_enable_cb_prepare with NAPI_STATE_SCHED set (under napi lock), later call virtqueue_poll with NAPI_STATE_SCHED clear (outside the lock).
Reported-by: Jason Wang <redacted> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> --- drivers/net/virtio_net.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
quoted hunk ↗ jump to hunk
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index 5305bd1..fbdd79a 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c@@ -622,8 +622,9 @@ again: /* Out of packets? */ if (received < budget) { + unsigned r = virtqueue_enable_cb_prepare(rq->vq);
Empty line wouldn't hurt here, after declaration.
WBR, Sergei