[PATCH v2] dmaengine: plx_dma: Fix potential deadlock on &plxdev->ring_lock

[Chengfeng Ye]

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 lock was changed from spin_lock_bh() to spin_lock() by a previous patch
for performance concern but unintentionally brought this potential deadlock
problem.

This patch reverts back to spin_lock_bh() to fix the deadlock problem.

Fixes: 1d05a0bdb420 ("dmaengine: plx_dma: Move spin_lock_bh() to spin_lock()")
Signed-off-by: Chengfeng Ye <dg573847474@xxxxxxxxx>

Changes in v2
- Consistently use spin_lock_bh() on &plxdev->ring_lock instead of
spin_lock_irqsave().
---
 drivers/dma/plx_dma.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/plx_dma.c b/drivers/dma/plx_dma.c
index 34b6416c3287..7693c067a1aa 100644
--- a/drivers/dma/plx_dma.c
+++ b/drivers/dma/plx_dma.c
@@ -137,7 +137,7 @@ static void plx_dma_process_desc(struct plx_dma_dev *plxdev)
 	struct plx_dma_desc *desc;
 	u32 flags;
 
-	spin_lock(&plxdev->ring_lock);
+	spin_lock_bh(&plxdev->ring_lock);
 
 	while (plxdev->tail != plxdev->head) {
 		desc = plx_dma_get_desc(plxdev, plxdev->tail);
@@ -165,7 +165,7 @@ static void plx_dma_process_desc(struct plx_dma_dev *plxdev)
 		plxdev->tail++;
 	}
 
-	spin_unlock(&plxdev->ring_lock);
+	spin_unlock_bh(&plxdev->ring_lock);
 }
 
 static void plx_dma_abort_desc(struct plx_dma_dev *plxdev)
-- 
2.17.1