[PATCH v2] dmaengine: sun6i: Fix potential deadlock on &sdev->lock

[Chengfeng Ye]

As &sdev->lock is acquired by tasklet sun6i_dma_tasklet() executed under
softirq context, other acquisition of the same lock under process context
should disable irq, otherwise deadlock could happen if the soft irq preempt
the execution while the lock is held in process context on the same CPU.

sun6i_dma_terminate_all() and sun6i_dma_pause() callbacks acquire the same
lock without disabling irq inside the function.

Possible deadlock scenario:
sun6i_dma_pause()
    -> spin_lock(&sdev->lock);
        <tasklet softirq interruption>
        -> sun6i_dma_tasklet()
        -> spin_lock_irq(&sdev->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() to
disable softirq.

Changelog:
v1 - >v2
- Use spin_lock_irqsave() instead of spin_lock_bh(), since outside caller
  could already call with bh disable, in the case spin_unlock_bh() would
  unintentionally enable bh.

Signed-off-by: Chengfeng Ye <dg573847474 at gmail.com>
---
 drivers/dma/sun6i-dma.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/dma/sun6i-dma.c b/drivers/dma/sun6i-dma.c
index ebfd29888b2f..30f426299703 100644
--- a/drivers/dma/sun6i-dma.c
+++ b/drivers/dma/sun6i-dma.c
@@ -866,6 +866,7 @@ static int sun6i_dma_pause(struct dma_chan *chan)
 	struct sun6i_dma_dev *sdev = to_sun6i_dma_dev(chan->device);
 	struct sun6i_vchan *vchan = to_sun6i_vchan(chan);
 	struct sun6i_pchan *pchan = vchan->phy;
+	unsigned long flags;
 
 	dev_dbg(chan2dev(chan), "vchan %p: pause\n", &vchan->vc);
 
@@ -873,9 +874,9 @@ static int sun6i_dma_pause(struct dma_chan *chan)
 		writel(DMA_CHAN_PAUSE_PAUSE,
 		       pchan->base + DMA_CHAN_PAUSE);
 	} else {
-		spin_lock(&sdev->lock);
+		spin_lock_irqsave(&sdev->lock, flags);
 		list_del_init(&vchan->node);
-		spin_unlock(&sdev->lock);
+		spin_unlock_irqrestore(&sdev->lock, flags);
 	}
 
 	return 0;
@@ -914,9 +915,9 @@ static int sun6i_dma_terminate_all(struct dma_chan *chan)
 	unsigned long flags;
 	LIST_HEAD(head);
 
-	spin_lock(&sdev->lock);
+	spin_lock_irqsave(&sdev->lock, flags);
 	list_del_init(&vchan->node);
-	spin_unlock(&sdev->lock);
+	spin_unlock_irqrestore(&sdev->lock, flags);
 
 	spin_lock_irqsave(&vchan->vc.lock, flags);
 
-- 
2.17.1