[PATCH v2 6/8] drm/panfrost: Use mutex_trylock in panfrost_gem_purge

[Rob Herring]

Lockdep reports a circular locking dependency with pages_lock taken in
the shrinker callback. The deadlock can't actually happen with current
users at least as a BO will never be purgeable when pages_lock is held.
To be safe, let's use mutex_trylock() instead and bail if a BO is locked
already.

Cc: Tomeu Vizoso <tomeu.vizoso at collabora.com>
Cc: Steven Price <steven.price at arm.com>
Cc: Alyssa Rosenzweig <alyssa.rosenzweig at collabora.com>
Cc: David Airlie <airlied at linux.ie>
Cc: Daniel Vetter <daniel at ffwll.ch>
Signed-off-by: Rob Herring <robh at kernel.org>
---
v2: new patch

 drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
index d191632b6197..458f0fa68111 100644
--- a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
+++ b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
@@ -36,15 +36,18 @@ panfrost_gem_shrinker_count(struct shrinker *shrinker, struct shrink_control *sc
 	return count;
 }
 
-static void panfrost_gem_purge(struct drm_gem_object *obj)
+static bool panfrost_gem_purge(struct drm_gem_object *obj)
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
-	mutex_lock(&shmem->pages_lock);
+
+	if (!mutex_trylock(&shmem->pages_lock))
+		return false;
 
 	panfrost_mmu_unmap(to_panfrost_bo(obj));
 	drm_gem_shmem_purge_locked(obj);
 
 	mutex_unlock(&shmem->pages_lock);
+	return true;
 }
 
 static unsigned long
@@ -61,8 +64,8 @@ panfrost_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
 	list_for_each_entry_safe(shmem, tmp, &pfdev->shrinker_list, madv_list) {
 		if (freed >= sc->nr_to_scan)
 			break;
-		if (drm_gem_shmem_is_purgeable(shmem)) {
-			panfrost_gem_purge(&shmem->base);
+		if (drm_gem_shmem_is_purgeable(shmem) &&
+		    panfrost_gem_purge(&shmem->base)) {
 			freed += shmem->base.size >> PAGE_SHIFT;
 			list_del_init(&shmem->madv_list);
 		}
-- 
2.20.1