drm/panfrost: Use mutex_trylock in panfrost_gem_purge

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@collabora.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823021216.5862-7-robh@kernel.org

diff --git a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
index d191632..458f0fa 100644 (file)
--- 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);
                }