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Reply-To: christian.koenig@amd.com
Subject: Re: [PATCH 1/3] locking/ww_mutex: cleanup lock->ctx usage in amdgpu
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From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>
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Date:   Mon, 19 Feb 2018 16:41:55 +0100
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Am 19.02.2018 um 16:24 schrieb Daniel Vetter:
> On Thu, Feb 15, 2018 at 03:19:42PM +0100, Christian König wrote:
>> amdgpu needs to verify if userspace sends us valid addresses and the simplest
>> way of doing this is to check if the buffer object is locked with the ticket
>> of the current submission.
>>
>> Clean up the access to the ww_mutex internals by providing a function
>> for this and extend the check to the thread owning the underlying mutex.
>>
>> Signed-off-by: Christian König <christian.koenig@amd.com>
>> ---
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c |  3 ++-
>>   include/linux/ww_mutex.h               | 17 +++++++++++++++++
>>   2 files changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
>> index eaa3cb0c3ad1..4c04b560e358 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
>> @@ -1594,7 +1594,8 @@ int amdgpu_cs_find_mapping(struct amdgpu_cs_parser *parser,
>>   	*map = mapping;
>>   
>>   	/* Double check that the BO is reserved by this CS */
>> -	if (READ_ONCE((*bo)->tbo.resv->lock.ctx) != &parser->ticket)
>> +	if (!ww_mutex_is_owned_by(&(*bo)->tbo.resv->lock, current,
>> +				  &parser->ticket))
>>   		return -EINVAL;
>>   
>>   	if (!((*bo)->flags & AMDGPU_GEM_CREATE_VRAM_CONTIGUOUS)) {
>> diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
>> index 39fda195bf78..dd580db289e8 100644
>> --- a/include/linux/ww_mutex.h
>> +++ b/include/linux/ww_mutex.h
>> @@ -358,4 +358,21 @@ static inline bool ww_mutex_is_locked(struct ww_mutex *lock)
>>   	return mutex_is_locked(&lock->base);
>>   }
>>   
>> +/**
>> + * ww_mutex_is_owned_by - is the w/w mutex locked by this task in that context
>> + * @lock: the mutex to be queried
>> + * @task: the task structure to check
>> + * @ctx: the w/w acquire context to test
>> + *
>> + * Returns true if the mutex is locked in the context by the given task, false
>> + * otherwise.
>> + */
>> +static inline bool ww_mutex_is_owned_by(struct ww_mutex *lock,
>> +					struct task_struct *task,
>> +					struct ww_acquire_ctx *ctx)
>> +{
>> +	return likely(__mutex_owner(&lock->base) == task) &&
>> +		READ_ONCE(lock->ctx) == ctx;
> Just comparing the context should be good enough. If you ever pass a
> ww_acquire_ctx which does not belong to your own thread your seriously
> wreaking things much worse already (and if we do catch that, should
> probably lock the ctx to a given task when ww-mutex debugging is enabled).
>
> That also simplifies the function signature.
>
> Of course that means if you don't have a ctx, you can't test ownership of
> a ww_mute, but I think that's not a really valid use-case.

Well exactly that is the use case in TTM, see patch #3 in this series.

In TTM the evicted BOs are trylocked and so we need a way of testing for 
ownership without a context.

Christian.

>   And not needed
> for cmd submission, where you need the ctx anyway.
>
> Besides this interface nit looks all good. With the task check&parameter
> removed:
>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>
> -Daniel
>
>> +}
>> +
>>   #endif
>> -- 
>> 2.14.1
>>
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