[PATCH] dmaengine: plx_dma: Fix potential deadlock on &plxdev->ring_lock

[Chengfeng Ye <dg573847474@gmail.com>]

As plx_dma_process_desc() is invoked by both tasklet plx_dma_desc_task()
under softirq context and plx_dma_tx_status() callback that executed under
process context, the lock aquicision of &plxdev->ring_lock inside
plx_dma_process_desc() should disable irq otherwise deadlock could happen
if the irq preempts the execution of process context code while the lock
is held in process context on the same CPU.

Possible deadlock scenario:
plx_dma_tx_status()
    -> plx_dma_process_desc()
    -> spin_lock(&plxdev->ring_lock)
        <tasklet softirq>
        -> plx_dma_desc_task()
        -> plx_dma_process_desc()
        -> spin_lock(&plxdev->ring_lock) (deadlock here)

This flaw was found by an experimental static analysis tool I am developing
for irq-related deadlock.

The tentative patch fixes the potential deadlock by spin_lock_irqsave() in
plx_dma_process_desc() to disable irq while lock is held.

Signed-off-by: Chengfeng Ye <dg573847474@gmail.com>
---
 drivers/dma/plx_dma.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/plx_dma.c b/drivers/dma/plx_dma.c
index 34b6416c3287..46068b8d83f9 100644
--- a/drivers/dma/plx_dma.c
+++ b/drivers/dma/plx_dma.c
@@ -135,9 +135,10 @@ static void plx_dma_process_desc(struct plx_dma_dev *plxdev)
 {
 	struct dmaengine_result res;
 	struct plx_dma_desc *desc;
+	unsigned long lock_flags;
 	u32 flags;
 
-	spin_lock(&plxdev->ring_lock);
+	spin_lock_irqsave(&plxdev->ring_lock, lock_flags);
 
 	while (plxdev->tail != plxdev->head) {
 		desc = plx_dma_get_desc(plxdev, plxdev->tail);
@@ -165,7 +166,7 @@ static void plx_dma_process_desc(struct plx_dma_dev *plxdev)
 		plxdev->tail++;
 	}
 
-	spin_unlock(&plxdev->ring_lock);
+	spin_unlock_irqrestore(&plxdev->ring_lock, lock_flags);
 }
 
 static void plx_dma_abort_desc(struct plx_dma_dev *plxdev)
-- 
2.17.1