drm/ttm: Use mutex_trylock() to avoid deadlock inside shrinker functions.

I can observe that RHEL7 environment stalls with 100% CPU usage when a
certain type of memory pressure is given. While the shrinker functions
are called by shrink_slab() before the OOM killer is triggered, the stall
lasts for many minutes.

One of reasons of this stall is that
ttm_dma_pool_shrink_count()/ttm_dma_pool_shrink_scan() are called and
are blocked at mutex_lock(&_manager->lock). GFP_KERNEL allocation with
_manager->lock held causes someone (including kswapd) to deadlock when
these functions are called due to memory pressure. This patch changes
"mutex_lock();" to "if (!mutex_trylock()) return ...;" in order to
avoid deadlock.
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: stable <stable@kernel.org> [3.3+]
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_page_alloc_dma.c

@@ -1014,7 +1014,8 @@ ttm_dma_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 	if (list_empty(&_manager->pools))
 		return SHRINK_STOP;
 
-	mutex_lock(&_manager->lock);
+	if (!mutex_trylock(&_manager->lock))
+		return SHRINK_STOP;
 	if (!_manager->npools)
 		goto out;
 	pool_offset = ++start_pool % _manager->npools;
@@ -1047,7 +1048,8 @@ ttm_dma_pool_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 	struct device_pools *p;
 	unsigned long count = 0;
 
-	mutex_lock(&_manager->lock);
+	if (!mutex_trylock(&_manager->lock))
+		return 0;
 	list_for_each_entry(p, &_manager->pools, pools)
 		count += p->pool->npages_free;
 	mutex_unlock(&_manager->lock);