[PATCH] drm/xe: Remove unwanted mutex locking

[PATCH] drm/xe: Remove unwanted mutex locking

[Niranjana Vishwanathapura]

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.

Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
---
 drivers/gpu/drm/xe/xe_device.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_device.c b/drivers/gpu/drm/xe/xe_device.c
index f04b11e45c2d..4cca16f2d4ed 100644
--- a/drivers/gpu/drm/xe/xe_device.c
+++ b/drivers/gpu/drm/xe/xe_device.c
@@ -96,12 +96,11 @@ 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 locking needed here */
 	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

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

[Matthew Brost]

On Wed, May 29, 2024 at 11:08:51AM -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.
> 

To be uniform, we should also drop the xef->vm.lock in this function
too. Can be done in a follow up.

Also mention that this lock protects the lookup plus taking a reference
which doesn't apply to this code path. Maybe also say if FD is closing,
IOCTLs presumably can't be modifying the xarray.

With an updated commit message:
Reviewed-by: Matthew Brost <matthew.brost@intel.com>

> Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_device.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_device.c b/drivers/gpu/drm/xe/xe_device.c
> index f04b11e45c2d..4cca16f2d4ed 100644
> --- a/drivers/gpu/drm/xe/xe_device.c
> +++ b/drivers/gpu/drm/xe/xe_device.c
> @@ -96,12 +96,11 @@ 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 locking needed here */
>  	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
>