[PATCH] dmaengine: xgene: Fix potential deadlock on &chan->lock

[Chengfeng Ye <dg573847474@gmail.com>]

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

Possible deadlock scenario:
xgene_dma_free_chan_resources()
    -> xgene_dma_cleanup_descriptors()
    -> spin_lock(&chan->lock)
        <tasklet softirq>
        -> xgene_dma_tasklet_cb()
        -> xgene_dma_cleanup_descriptors()
        -> spin_lock(&chan->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/xgene-dma.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/xgene-dma.c b/drivers/dma/xgene-dma.c
index 3589b4ef50b8..e766511badcf 100644
--- a/drivers/dma/xgene-dma.c
+++ b/drivers/dma/xgene-dma.c
@@ -689,11 +689,12 @@ static void xgene_dma_cleanup_descriptors(struct xgene_dma_chan *chan)
 	struct xgene_dma_desc_sw *desc_sw, *_desc_sw;
 	struct xgene_dma_desc_hw *desc_hw;
 	struct list_head ld_completed;
+	unsigned long flags;
 	u8 status;
 
 	INIT_LIST_HEAD(&ld_completed);
 
-	spin_lock(&chan->lock);
+	spin_lock_irqsave(&chan->lock, flags);
 
 	/* Clean already completed and acked descriptors */
 	xgene_dma_clean_completed_descriptor(chan);
@@ -762,7 +763,7 @@ static void xgene_dma_cleanup_descriptors(struct xgene_dma_chan *chan)
 	 */
 	xgene_chan_xfer_ld_pending(chan);
 
-	spin_unlock(&chan->lock);
+	spin_unlock_irqrestore(&chan->lock, flags);
 
 	/* Run the callback for each descriptor, in order */
 	list_for_each_entry_safe(desc_sw, _desc_sw, &ld_completed, node) {
-- 
2.17.1