[PATCH 2/8] drm/panfrost: Fix a race in panfrost_ioctl_madvise()

[Steven Price]

On 29/11/2019 14:33, Boris Brezillon wrote:
> On Fri, 29 Nov 2019 14:24:48 +0000
> Steven Price <steven.price at arm.com> wrote:
> 
>> On 29/11/2019 13:59, Boris Brezillon wrote:
>>> If 2 threads change the MADVISE property of the same BO in parallel we
>>> might end up with an shmem->madv value that's inconsistent with the
>>> presence of the BO in the shrinker list.  
>>
>> I'm a bit worried from the point of view of user space sanity that you
>> observed this - but clearly the kernel should be robust!
> 
> It's not something I observed, just found the race by inspecting the
> code, and I thought it was worth fixing it.

Good! ;) Your cover letter referring to a "debug session" made me think
you'd actually hit all these.

Steve

>>
>>>
>>> The easiest solution to fix that is to protect the
>>> drm_gem_shmem_madvise() call with the shrinker lock.
>>>
>>> Fixes: 013b65101315 ("drm/panfrost: Add madvise and shrinker support")
>>> Cc: <stable at vger.kernel.org>
>>> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>  
>>
>> Reviewed-by: Steven Price <steven.price at arm.com>
> 
> Thanks.
> 
>>
>>> ---
>>>  drivers/gpu/drm/panfrost/panfrost_drv.c | 9 ++++-----
>>>  1 file changed, 4 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
>>> index f21bc8a7ee3a..efc0a24d1f4c 100644
>>> --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
>>> +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
>>> @@ -347,20 +347,19 @@ static int panfrost_ioctl_madvise(struct drm_device *dev, void *data,
>>>  		return -ENOENT;
>>>  	}
>>>  
>>> +	mutex_lock(&pfdev->shrinker_lock);
>>>  	args->retained = drm_gem_shmem_madvise(gem_obj, args->madv);
>>>  
>>>  	if (args->retained) {
>>>  		struct panfrost_gem_object *bo = to_panfrost_bo(gem_obj);
>>>  
>>> -		mutex_lock(&pfdev->shrinker_lock);
>>> -
>>>  		if (args->madv == PANFROST_MADV_DONTNEED)
>>> -			list_add_tail(&bo->base.madv_list, &pfdev->shrinker_list);
>>> +			list_add_tail(&bo->base.madv_list,
>>> +				      &pfdev->shrinker_list);
>>>  		else if (args->madv == PANFROST_MADV_WILLNEED)
>>>  			list_del_init(&bo->base.madv_list);
>>> -
>>> -		mutex_unlock(&pfdev->shrinker_lock);
>>>  	}
>>> +	mutex_unlock(&pfdev->shrinker_lock);
>>>  
>>>  	drm_gem_object_put_unlocked(gem_obj);
>>>  	return 0;
>>>   
>>
> 
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