drm/xe: Remove unwanted mutex locking

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)
v4: Further expand code comment (Rodirgo Vivi)

Signed-off-by: Niranjana Vishwanathapura <[e-mail]>
Reviewed-by: Matthew Brost <[e-mail]>
Reviewed-by: Jagmeet Randhawa <[e-mail]>
Acked-by: Rodrigo Vivi <[e-mail]>
Signed-off-by: Matthew Brost <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/xe/xe_device.c

···
 	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 for it
+	 * when FD is closing as 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);