[PATCH 3/4] thunderbolt: stream: Do not hold the lock while busy polling

[Mika Westerberg <mika.westerberg@linux.intel.com>]

When in busy poll mode with a reader and writer, and there is nothing
completed on the rings both sides keep taking and releasing the lock
from each other and this can cause stalls. Avoid this by calling the new
API function tb_ring_poll_pending() that only returns true if there is
anything available in which case they can take the lock and know there
is now work to do.

Reported-by: Alan Borzeszkowski <alan.borzeszkowski@linux.intel.com>
Assisted-by: LLM
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
---
 drivers/thunderbolt/stream.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/drivers/thunderbolt/stream.c b/drivers/thunderbolt/stream.c
index 7b618d8a7d82..b4f3b1db9d82 100644
--- a/drivers/thunderbolt/stream.c
+++ b/drivers/thunderbolt/stream.c
@@ -734,6 +734,22 @@ static int tbstream_dev_lock(struct tbstream_dev *sdev, bool nowait)
 	return 0;
 }
 
+/* Must not be called with @sdev->lock held */
+static int tbstream_dev_busy_poll_wait(struct tbstream_dev *sdev,
+				       struct tbstream_ring *ring)
+{
+	for (;;) {
+		if (signal_pending(current))
+			return -ERESTARTSYS;
+		if (tb_ring_poll_pending(ring->ring))
+			return 0;
+		if (tbstream_dev_valid(sdev) != 0 ||
+		    tbstream_dev_closed(sdev) || tbstream_dev_removed(sdev))
+			return 0;
+		cond_resched();
+	}
+}
+
 static ssize_t
 tbstream_dev_fops_read_iter(struct kiocb *kiocb, struct iov_iter *to)
 {
@@ -777,9 +793,9 @@ tbstream_dev_fops_read_iter(struct kiocb *kiocb, struct iov_iter *to)
 			return -EAGAIN;
 
 		if (sdev->busy_poll) {
-			if (signal_pending(current))
-				return -ERESTARTSYS;
-			cond_resched();
+			ret = tbstream_dev_busy_poll_wait(sdev, &sdev->rx_ring);
+			if (ret)
+				return ret;
 		} else {
 			ret = wait_event_interruptible(sdev->wait,
 					READ_ONCE(sdev->rx_pending) ||
@@ -891,9 +907,9 @@ tbstream_dev_fops_write_iter(struct kiocb *kiocb, struct iov_iter *from)
 			return -EAGAIN;
 
 		if (sdev->busy_poll) {
-			if (signal_pending(current))
-				return -ERESTARTSYS;
-			cond_resched();
+			ret = tbstream_dev_busy_poll_wait(sdev, &sdev->tx_ring);
+			if (ret)
+				return ret;
 		} else {
 			ret = wait_event_interruptible(sdev->wait,
 					tbstream_ring_available(&sdev->tx_ring) ||
-- 
2.50.1