Re: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

[Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>]

On 2018/08/24 22:52, Michal Hocko wrote:
> @@ -180,11 +180,15 @@ void amdgpu_mn_unlock(struct amdgpu_mn *mn)
>   */
>  static int amdgpu_mn_read_lock(struct amdgpu_mn *amn, bool blockable)
>  {
> -	if (blockable)
> -		mutex_lock(&amn->read_lock);
> -	else if (!mutex_trylock(&amn->read_lock))
> -		return -EAGAIN;
> -
> +	/*
> +	 * We can take sleepable lock even on !blockable mode because
> +	 * read_lock is only ever take from this path and the notifier
> +	 * lock never really sleeps. In fact the only reason why the
> +	 * later is sleepable is because the notifier itself might sleep
> +	 * in amdgpu_mn_invalidate_node but blockable mode is handled
> +	 * before calling into that path.
> +	 */
> +	mutex_lock(&amn->read_lock);
>  	if (atomic_inc_return(&amn->recursion) == 1)
>  		down_read_non_owner(&amn->lock);
>  	mutex_unlock(&amn->read_lock);
> 

I'm not following. Why don't we need to do like below (given that
nobody except amdgpu_mn_read_lock() holds ->read_lock) because e.g.
drm_sched_fence_create() from drm_sched_job_init() from amdgpu_cs_submit()
is doing GFP_KERNEL memory allocation with ->lock held for write?

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
index e55508b..e1cb344 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
@@ -64,8 +64,6 @@
  * @node: hash table node to find structure by adev and mn
  * @lock: rw semaphore protecting the notifier nodes
  * @objects: interval tree containing amdgpu_mn_nodes
- * @read_lock: mutex for recursive locking of @lock
- * @recursion: depth of recursion
  *
  * Data for each amdgpu device and process address space.
  */
@@ -85,8 +83,6 @@ struct amdgpu_mn {
 	/* objects protected by lock */
 	struct rw_semaphore	lock;
 	struct rb_root_cached	objects;
-	struct mutex		read_lock;
-	atomic_t		recursion;
 };
 
 /**
@@ -181,14 +177,9 @@ void amdgpu_mn_unlock(struct amdgpu_mn *mn)
 static int amdgpu_mn_read_lock(struct amdgpu_mn *amn, bool blockable)
 {
 	if (blockable)
-		mutex_lock(&amn->read_lock);
-	else if (!mutex_trylock(&amn->read_lock))
+		down_read(&amn->lock);
+	else if (!down_read_trylock(&amn->lock))
 		return -EAGAIN;
-
-	if (atomic_inc_return(&amn->recursion) == 1)
-		down_read_non_owner(&amn->lock);
-	mutex_unlock(&amn->read_lock);
-
 	return 0;
 }
 
@@ -199,8 +190,7 @@ static int amdgpu_mn_read_lock(struct amdgpu_mn *amn, bool blockable)
  */
 static void amdgpu_mn_read_unlock(struct amdgpu_mn *amn)
 {
-	if (atomic_dec_return(&amn->recursion) == 0)
-		up_read_non_owner(&amn->lock);
+	up_read(&amn->lock);
 }
 
 /**
@@ -410,8 +400,6 @@ struct amdgpu_mn *amdgpu_mn_get(struct amdgpu_device *adev,
 	amn->type = type;
 	amn->mn.ops = &amdgpu_mn_ops[type];
 	amn->objects = RB_ROOT_CACHED;
-	mutex_init(&amn->read_lock);
-	atomic_set(&amn->recursion, 0);
 
 	r = __mmu_notifier_register(&amn->mn, mm);
 	if (r)