[PATCH 1/1] rpmsg: virtio_rpmsg_bus - prevent possible race condition

[Tim Blechmann <tim.blechmann@gmail.com>]

When we cannot get a tx buffer (`get_a_tx_buf`) `rpmsg_upref_sleepers`
enables tx-complete interrupt.
However if the interrupt is executed after `get_a_tx_buf` and before
`rpmsg_upref_sleepers` we may miss the tx-complete interrupt and sleep
for the full 15 seconds. To avoid this race condition, we re-try once
before we really start to sleep.

Signed-off-by: Tim Blechmann <tim@klingt.org>
---
 drivers/rpmsg/virtio_rpmsg_bus.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_bus.c
index 905ac7910c98..2a9d42225e60 100644
--- a/drivers/rpmsg/virtio_rpmsg_bus.c
+++ b/drivers/rpmsg/virtio_rpmsg_bus.c
@@ -587,21 +587,27 @@ static int rpmsg_send_offchannel_raw(struct rpmsg_device *rpdev,
 
 	/* no free buffer ? wait for one (but bail after 15 seconds) */
 	while (!msg) {
 		/* enable "tx-complete" interrupts, if not already enabled */
 		rpmsg_upref_sleepers(vrp);
 
-		/*
-		 * sleep until a free buffer is available or 15 secs elapse.
-		 * the timeout period is not configurable because there's
-		 * little point in asking drivers to specify that.
-		 * if later this happens to be required, it'd be easy to add.
-		 */
-		err = wait_event_interruptible_timeout(vrp->sendq,
-					(msg = get_a_tx_buf(vrp)),
-					msecs_to_jiffies(15000));
+		/* make sure to retry to grab tx buffer before we start waiting */
+		msg = get_a_tx_buf(vrp);
+		if (msg) {
+			err = 0;
+		} else {
+			/*
+			 * sleep until a free buffer is available or 15 secs elapse.
+			 * the timeout period is not configurable because there's
+			 * little point in asking drivers to specify that.
+			 * if later this happens to be required, it'd be easy to add.
+			 */
+			err = wait_event_interruptible_timeout(vrp->sendq,
+						(msg = get_a_tx_buf(vrp)),
+						msecs_to_jiffies(15000));
+		}
 
 		/* disable "tx-complete" interrupts if we're the last sleeper */
 		rpmsg_downref_sleepers(vrp);
 
 		/* timeout ? */
 		if (!err) {
-- 
2.34.1