Re: [PATCH v3] drm/xe: Remove unwanted mutex locking

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Wed, May 29, 2024 at 02:39:25PM -0700, Niranjana Vishwanathapura wrote:
> Do not hold xef->exec_queue.lock mutex while parsing the xarray
> xef->exec_queue.xa in xe_file_close() as it is not needed and
> will cause an unwanted dependency between this lock and the vm->lock.
> 
> This lock protects the exec queue lookup and reference taking which
> doesn't apply to this code path. When FD is closing, IOCTLs presumably
> can't be modifying the xarray.
> 
> v2: Update commit text (Matt Brost)
> v3: Add more code comment (Rodrigo Vivi)
> 
> Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
> Reviewed-by: Matthew Brost <matthew.brost@intel.com>
> Reviewed-by: Jagmeet Randhawa <jagmeet.randhawa@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_device.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_device.c b/drivers/gpu/drm/xe/xe_device.c
> index f04b11e45c2d..bf4c4306f6d3 100644
> --- a/drivers/gpu/drm/xe/xe_device.c
> +++ b/drivers/gpu/drm/xe/xe_device.c
> @@ -96,12 +96,15 @@ static void xe_file_close(struct drm_device *dev, struct drm_file *file)
>  	struct xe_exec_queue *q;
>  	unsigned long idx;
>  
> -	mutex_lock(&xef->exec_queue.lock);
> +	/*
> +	 * No need for exec_queue.lock here as there is no contention at
> +	 * this point.

I'm really sorry to be pedantic here. "But why is there no contention at this point?"
This remain unanswered here in the comment, and only a further code inspection or
a git-blame archeology would lead to the commit message with:
"When FD is closing, IOCTLs presumably can't be modifying the xarray."

>  Taking exec_queue.lock here causes undue dependency
> +	 * on vm->lock taken during xe_exec_queue_kill().
> +	 */
>  	xa_for_each(&xef->exec_queue.xa, idx, q) {
>  		xe_exec_queue_kill(q);
>  		xe_exec_queue_put(q);
>  	}
> -	mutex_unlock(&xef->exec_queue.lock);
>  	xa_destroy(&xef->exec_queue.xa);
>  	mutex_destroy(&xef->exec_queue.lock);
>  	mutex_lock(&xef->vm.lock);
> -- 
> 2.43.0
>