linux-kernel - Re: [PATCH] gpu: drm: remove redundant dma_fence_put() when drm_sched_job_add

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Date:   Tue, 26 Apr 2022 10:54:56 +0800
From:   Hangyu Hua <hbh25y@...il.com>
To:     Andrey Grodzovsky <andrey.grodzovsky@....com>, yuq825@...il.com,
        airlied@...ux.ie, daniel@...ll.ch
Cc:     dri-devel@...ts.freedesktop.org, lima@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gpu: drm: remove redundant dma_fence_put() when
 drm_sched_job_add_dependency() fails

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@...il.com>
>> ---
>>   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. 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()

Thanks,
Hangyu

> 
>>               return ret;
>>           }
>>       }