Re: [PATCH 1/3] locking/ww_mutex: cleanup lock->ctx usage in amdgpu

[Daniel Vetter <daniel@ffwll.ch>]

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. 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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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch