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Subject[PATCH] dmaengine: pl330: fix descriptor allocation fail
Date
Thread A calls pl330_get_desc() to get descriptor. If DMAC descriptor
pool is empty pl330_get_desc() allocates new descriptor using add_desc()
and then get newly allocated descriptor using pluck_desc().
It is possible that another concurrent thread B calls pluck_desc()
and catch newly allocated descriptor. In that case descriptor allocation
for thread A will fail with:

kernel: dma-pl330 20078000.dma-controller: pl330_get_desc:2469 ALERT!

The commit fix the issue by calling _add_desc() to allocate new descriptor
to the local on stack pool and than get it from local pool. So the issue
described will nether happen.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
---
drivers/dma/pl330.c | 44 +++++++++++++++++++++++++++-----------------
1 file changed, 27 insertions(+), 17 deletions(-)

diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
index f37f497..0e7f6c9 100644
--- a/drivers/dma/pl330.c
+++ b/drivers/dma/pl330.c
@@ -2417,7 +2417,8 @@ static inline void _init_desc(struct dma_pl330_desc *desc)
}

/* Returns the number of descriptors added to the DMAC pool */
-static int add_desc(struct pl330_dmac *pl330, gfp_t flg, int count)
+static int _add_desc(struct list_head *pool, spinlock_t *lock,
+ gfp_t flg, int count)
{
struct dma_pl330_desc *desc;
unsigned long flags;
@@ -2427,27 +2428,33 @@ static int add_desc(struct pl330_dmac *pl330, gfp_t flg, int count)
if (!desc)
return 0;

- spin_lock_irqsave(&pl330->pool_lock, flags);
+ spin_lock_irqsave(lock, flags);

for (i = 0; i < count; i++) {
_init_desc(&desc[i]);
- list_add_tail(&desc[i].node, &pl330->desc_pool);
+ list_add_tail(&desc[i].node, pool);
}

- spin_unlock_irqrestore(&pl330->pool_lock, flags);
+ spin_unlock_irqrestore(lock, flags);

return count;
}

-static struct dma_pl330_desc *pluck_desc(struct pl330_dmac *pl330)
+static int add_desc(struct pl330_dmac *pl330, gfp_t flg, int count)
+{
+ return _add_desc(&pl330->desc_pool, &pl330->pool_lock, flg, count);
+}
+
+static struct dma_pl330_desc *_pluck_desc(struct list_head *pool,
+ spinlock_t *lock)
{
struct dma_pl330_desc *desc = NULL;
unsigned long flags;

- spin_lock_irqsave(&pl330->pool_lock, flags);
+ spin_lock_irqsave(lock, flags);

- if (!list_empty(&pl330->desc_pool)) {
- desc = list_entry(pl330->desc_pool.next,
+ if (!list_empty(pool)) {
+ desc = list_entry(pool->next,
struct dma_pl330_desc, node);

list_del_init(&desc->node);
@@ -2456,11 +2463,16 @@ static struct dma_pl330_desc *pluck_desc(struct pl330_dmac *pl330)
desc->txd.callback = NULL;
}

- spin_unlock_irqrestore(&pl330->pool_lock, flags);
+ spin_unlock_irqrestore(lock, flags);

return desc;
}

+static struct dma_pl330_desc *pluck_desc(struct pl330_dmac *pl330)
+{
+ return _pluck_desc(&pl330->desc_pool, &pl330->pool_lock);
+}
+
static struct dma_pl330_desc *pl330_get_desc(struct dma_pl330_chan *pch)
{
struct pl330_dmac *pl330 = pch->dmac;
@@ -2472,16 +2484,14 @@ static struct dma_pl330_desc *pl330_get_desc(struct dma_pl330_chan *pch)

/* If the DMAC pool is empty, alloc new */
if (!desc) {
- if (!add_desc(pl330, GFP_ATOMIC, 1))
- return NULL;
+ DEFINE_SPINLOCK(lock);
+ LIST_HEAD(pool);

- /* Try again */
- desc = pluck_desc(pl330);
- if (!desc) {
- dev_err(pch->dmac->ddma.dev,
- "%s:%d ALERT!\n", __func__, __LINE__);
+ if (!_add_desc(&pool, &lock, GFP_ATOMIC, 1))
return NULL;
- }
+
+ desc = _pluck_desc(&pool, &lock);
+ WARN_ON(!desc || !list_empty(&pool));
}

/* Initialize the descriptor */
--
1.7.9.5
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