[PATCH] spi: fix one potential spin_lock issue

[Huiquan Zhong <huiquan.zhong@xxxxxxxxx>]

master->pump_messages workqueue can be queued by spi_start_queue(),
__spi_queued_transfer().

there is one case that if one resume thread call spi_start_queue(),
and at the same time another spi_device thread call spi_queued_transfer()
to do spi transfer, then the first workqueue will start the spi transfer,
but the next workqueue queued before SPI transfer complete, will unusual
terminate spi transfer.

Add spin_lock protection to avoid this case.

Signed-off-by: Huiquan Zhong <huiquan.zhong@xxxxxxxxx>
---
 drivers/spi/spi.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index d5d7d22..2bc387b 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -1017,9 +1017,11 @@ static int spi_start_queue(struct spi_master *master)
 
 	master->running = true;
 	master->cur_msg = NULL;
-	spin_unlock_irqrestore(&master->queue_lock, flags);
 
-	queue_kthread_work(&master->kworker, &master->pump_messages);
+	if (!list_empty(&master->queue))
+		queue_kthread_work(&master->kworker, &master->pump_messages);
+
+	spin_unlock_irqrestore(&master->queue_lock, flags);
 
 	return 0;
 }
-- 
1.7.9.5

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