Re: [PATCH] gpu: drm: remove redundant dma_fence_put() when drm_sched_job_add_dependency() fails

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On 2022/4/26 22:55, Andrey Grodzovsky wrote:

On 2022-04-25 22:54, Hangyu Hua wrote:
On 2022/4/25 23:42, Andrey Grodzovsky wrote:
On 2022-04-25 04:36, Hangyu Hua wrote:

When drm_sched_job_add_dependency() fails, dma_fence_put() will be called internally. Calling it again after drm_sched_job_add_dependency() finishes
may result in a dangling pointer.

Fix this by removing redundant dma_fence_put().

Signed-off-by: Hangyu Hua <hbh25y@xxxxxxxxx>
---
  drivers/gpu/drm/lima/lima_gem.c        | 1 -
  drivers/gpu/drm/scheduler/sched_main.c | 1 -
  2 files changed, 2 deletions(-)

diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
index 55bb1ec3c4f7..99c8e7f6bb1c 100644
--- a/drivers/gpu/drm/lima/lima_gem.c
+++ b/drivers/gpu/drm/lima/lima_gem.c
@@ -291,7 +291,6 @@ static int lima_gem_add_deps(struct drm_file *file, struct lima_submit *submit)           err = drm_sched_job_add_dependency(&submit->task->base, fence);
          if (err) {
-            dma_fence_put(fence);
              return err;


Makes sense here


          }
      }
diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
index b81fceb0b8a2..ebab9eca37a8 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -708,7 +708,6 @@ int drm_sched_job_add_implicit_dependencies(struct drm_sched_job *job,
          dma_fence_get(fence);
          ret = drm_sched_job_add_dependency(job, fence);
          if (ret) {
-            dma_fence_put(fence);



Not sure about this one since if you look at the relevant commits -
'drm/scheduler: fix drm_sched_job_add_implicit_dependencies' and
'drm/scheduler: fix drm_sched_job_add_implicit_dependencies harder'
You will see that the dma_fence_put here balances the extra dma_fence_get
above

Andrey


I don't think so. I checked the call chain and found no additional dma_fence_get(). But dma_fence_get() needs to be called before drm_sched_job_add_dependency() to keep the counter balanced.


I don't say there is an additional get, I just say that drm_sched_job_add_dependency doesn't grab an extra reference to the fences it stores so this needs to be done outside and for that drm_sched_job_add_implicit_dependencies->dma_fence_get is called and, if this addition fails you just call dma_fence_put to keep the counter balanced.


drm_sched_job_add_implicit_dependencies() will call drm_sched_job_add_dependency(). And drm_sched_job_add_dependency() already call dma_fence_put() when it fails. Calling dma_fence_put() twice doesn't make sense.

dma_fence_get() is in [2]. But dma_fence_put() will be called in [1] and [3] when xa_alloc() fails.


int drm_sched_job_add_dependency(struct drm_sched_job *job,
				 struct dma_fence *fence)
{
	...
	ret = xa_alloc(&job->dependencies, &id, fence, xa_limit_32b, GFP_KERNEL);
	if (ret != 0)
		dma_fence_put(fence);	<--- [1]

	return ret;
}
EXPORT_SYMBOL(drm_sched_job_add_dependency);


int drm_sched_job_add_implicit_dependencies(struct drm_sched_job *job,
					    struct drm_gem_object *obj,
					    bool write)
{
	struct dma_resv_iter cursor;
	struct dma_fence *fence;
	int ret;

	dma_resv_for_each_fence(&cursor, obj->resv, write, fence) {
		/* Make sure to grab an additional ref on the added fence */
		dma_fence_get(fence);	<--- [2]
		ret = drm_sched_job_add_dependency(job, fence);
		if (ret) {
			dma_fence_put(fence);	<--- [3]
			return ret;
		}
	}
	return 0;
}



On the other hand, dma_fence_get() and dma_fence_put() are meaningless here if threre is an extra dma_fence_get() beacause counter will not decrease to 0 during drm_sched_job_add_dependency().

I check the call chain as follows:

msm_ioctl_gem_submit()
-> submit_fence_sync()
-> drm_sched_job_add_implicit_dependencies()


Can you maybe trace or print one such example of problematic refcount that you are trying to fix ? I still don't see where is the problem.

Andrey


I also wish I could. System logs can make this easy. But i don't have a corresponding GPU physical device. drm_sched_job_add_implicit_dependencies is only used in a few devices.

Thanks.


Thanks,
Hangyu


              return ret;
          }
      }



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