[PATCH] dmaengine: pl330: fix double lock

[iari]

From: Iago Abal <mail@xxxxxxxxxxx>

The static bug finder EBA (http://www.iagoabal.eu/eba/) reported the
following double-lock bug:

    Double lock:
    1. spin_lock_irqsave(pch->lock, flags) at pl330_free_chan_resources:2236;
    2. call to function `pl330_release_channel' immediately after;
    3. call to function `dma_pl330_rqcb' in line 1753;
    4. spin_lock_irqsave(pch->lock, flags) at dma_pl330_rqcb:1505.

I have fixed it as suggested by Marek Szyprowski.

First, I have replaced `pch->lock' with `pl330->lock' in functions
`pl330_alloc_chan_resources' and `pl330_free_chan_resources'. This avoids
the double-lock by acquiring a different lock than `dma_pl330_rqcb'.

NOTE that, as a result, `pl330_free_chan_resources' executes
`list_splice_tail_init' on `pch->work_list' under lock `pl330->lock',
whereas in the rest of the code `pch->work_list' is protected by
`pch->lock'. I don't know if this may cause race conditions. Similarly
`pch->cyclic' is written by `pl330_alloc_chan_resources' under
`pl330->lock' but read by `pl330_tx_submit' under `pch->lock'.

Second, I have removed locking from `pl330_request_channel' and
`pl330_release_channel' functions. Function `pl330_request_channel' is
only called from `pl330_alloc_chan_resources', so the lock is already
held. Function `pl330_release_channel' is called from
`pl330_free_chan_resources', which already holds the lock, and from
`pl330_del'. Function `pl330_del' is called in an error path of
`pl330_probe' and at the end of `pl330_remove', but I assume that there
cannot be concurrent accesses to the protected data at those points.

Signed-off-by: Iago Abal <mail@xxxxxxxxxxx>
---
 drivers/dma/pl330.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
index 87fd015..3370e56 100644
--- a/drivers/dma/pl330.c
+++ b/drivers/dma/pl330.c
@@ -1696,7 +1696,6 @@ static bool _chan_ns(const struct pl330_dmac *pl330, int i)
 static struct pl330_thread *pl330_request_channel(struct pl330_dmac *pl330)
 {
 	struct pl330_thread *thrd = NULL;
-	unsigned long flags;
 	int chans, i;
 
 	if (pl330->state == DYING)
@@ -1704,8 +1703,6 @@ static struct pl330_thread *pl330_request_channel(struct pl330_dmac *pl330)
 
 	chans = pl330->pcfg.num_chan;
 
-	spin_lock_irqsave(&pl330->lock, flags);
-
 	for (i = 0; i < chans; i++) {
 		thrd = &pl330->channels[i];
 		if ((thrd->free) && (!_manager_ns(thrd) ||
@@ -1723,8 +1720,6 @@ static struct pl330_thread *pl330_request_channel(struct pl330_dmac *pl330)
 		thrd = NULL;
 	}
 
-	spin_unlock_irqrestore(&pl330->lock, flags);
-
 	return thrd;
 }
 
@@ -1742,7 +1737,6 @@ static inline void _free_event(struct pl330_thread *thrd, int ev)
 static void pl330_release_channel(struct pl330_thread *thrd)
 {
 	struct pl330_dmac *pl330;
-	unsigned long flags;
 
 	if (!thrd || thrd->free)
 		return;
@@ -1754,10 +1748,8 @@ static void pl330_release_channel(struct pl330_thread *thrd)
 
 	pl330 = thrd->dmac;
 
-	spin_lock_irqsave(&pl330->lock, flags);
 	_free_event(thrd, thrd->ev);
 	thrd->free = true;
-	spin_unlock_irqrestore(&pl330->lock, flags);
 }
 
 /* Initialize the structure for PL330 configuration, that can be used
@@ -2117,20 +2109,20 @@ static int pl330_alloc_chan_resources(struct dma_chan *chan)
 	struct pl330_dmac *pl330 = pch->dmac;
 	unsigned long flags;
 
-	spin_lock_irqsave(&pch->lock, flags);
+	spin_lock_irqsave(&pl330->lock, flags);
 
 	dma_cookie_init(chan);
 	pch->cyclic = false;
 
 	pch->thread = pl330_request_channel(pl330);
 	if (!pch->thread) {
-		spin_unlock_irqrestore(&pch->lock, flags);
+		spin_unlock_irqrestore(&pl330->lock, flags);
 		return -ENOMEM;
 	}
 
 	tasklet_init(&pch->task, pl330_tasklet, (unsigned long) pch);
 
-	spin_unlock_irqrestore(&pch->lock, flags);
+	spin_unlock_irqrestore(&pl330->lock, flags);
 
 	return 1;
 }
@@ -2228,12 +2220,13 @@ static int pl330_pause(struct dma_chan *chan)
 static void pl330_free_chan_resources(struct dma_chan *chan)
 {
 	struct dma_pl330_chan *pch = to_pchan(chan);
+	struct pl330_dmac *pl330 = pch->dmac;
 	unsigned long flags;
 
 	tasklet_kill(&pch->task);
 
 	pm_runtime_get_sync(pch->dmac->ddma.dev);
-	spin_lock_irqsave(&pch->lock, flags);
+	spin_lock_irqsave(&pl330->lock, flags);
 
 	pl330_release_channel(pch->thread);
 	pch->thread = NULL;
@@ -2241,7 +2234,7 @@ static void pl330_free_chan_resources(struct dma_chan *chan)
 	if (pch->cyclic)
 		list_splice_tail_init(&pch->work_list, &pch->dmac->desc_pool);
 
-	spin_unlock_irqrestore(&pch->lock, flags);
+	spin_unlock_irqrestore(&pl330->lock, flags);
 	pm_runtime_mark_last_busy(pch->dmac->ddma.dev);
 	pm_runtime_put_autosuspend(pch->dmac->ddma.dev);
 }
-- 
1.9.1