[PATCH 2/2] drm/amdgpu/userq: hold the doorbell xa lock during hang reset

[Junrui Luo via B4 Relay]

From: Junrui Luo <moonafterrain@xxxxxxxxxxx>

mes_userq_detect_and_reset() walks adev->userq_doorbell_xa with a bare
xa_for_each() and dereferences every entry: it reads queue->queue_type
and queue->doorbell_index, writes queue->state, and passes the queue to
amdgpu_userq_fence_driver_force_completion().  That xarray is device
wide, so most entries belong to other drm_files.

Nothing keeps those queues alive for the walk.  The caller,
amdgpu_userq_mgr_reset_work(), holds no lock, and amdgpu_mes_lock() is
dropped before the walk begins.  Meanwhile amdgpu_userq_destroy() erases
the doorbell entry via amdgpu_userq_cleanup() and kfree()s the queue
after dropping its own uq_mgr->userq_mutex; that per-file mutex cannot
cover another file's queue.  xa_for_each() releases its internal RCU read
lock before returning each entry, so the pointer can already be dangling
when the loop body touches it.

Fix by holding xa_lock_irqsave() across the walk, as
amdgpu_userq_process_fence_irq() and amdgpu_userq_mgr_cancel_reset_work()
already do.

Fixes: 54d18bc6003f ("drm/amdgpu/userq: add a detect and reset callback")
Reported-by: Yuhao Jiang <danisjiang@xxxxxxxxx>
Assisted-by: Claude:claude-opus-5
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Junrui Luo <moonafterrain@xxxxxxxxxxx>
---
Found by code inspection; not tested on hardware.
---
 drivers/gpu/drm/amd/amdgpu/mes_userqueue.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/mes_userqueue.c b/drivers/gpu/drm/amd/amdgpu/mes_userqueue.c
index 4e44a581a78a..f4d12e4b2d48 100644
--- a/drivers/gpu/drm/amd/amdgpu/mes_userqueue.c
+++ b/drivers/gpu/drm/amd/amdgpu/mes_userqueue.c
@@ -208,7 +208,7 @@ static int mes_userq_detect_and_reset(struct amdgpu_device *adev,
 	struct mes_detect_and_reset_queue_input input;
 	struct amdgpu_usermode_queue *queue;
 	unsigned int hung_db_num = 0;
-	unsigned long queue_id;
+	unsigned long queue_id, flags;
 	u32 db_array[8];
 	bool found_hung_queue = false;
 	int r, i;
@@ -230,6 +230,13 @@ static int mes_userq_detect_and_reset(struct amdgpu_device *adev,
 	if (r) {
 		dev_err(adev->dev, "Failed to detect and reset queues, err (%d)\n", r);
 	} else if (hung_db_num) {
+		/*
+		 * The doorbell xarray is device wide, so this walks queues
+		 * owned by other drm_files too. Hold its lock: the free path
+		 * erases the entry under the same lock strictly before it
+		 * frees the queue, so an entry found here stays allocated.
+		 */
+		xa_lock_irqsave(&adev->userq_doorbell_xa, flags);
 		xa_for_each(&adev->userq_doorbell_xa, queue_id, queue) {
 			if (queue->queue_type == queue_type) {
 				for (i = 0; i < hung_db_num; i++) {
@@ -238,14 +245,16 @@ static int mes_userq_detect_and_reset(struct amdgpu_device *adev,
 						found_hung_queue = true;
 						atomic_inc(&adev->gpu_reset_counter);
 						amdgpu_userq_fence_driver_force_completion(queue);
-						drm_dev_wedged_event(adev_to_drm(adev), DRM_WEDGE_RECOVERY_NONE, NULL);
 					}
 				}
 			}
 		}
+		xa_unlock_irqrestore(&adev->userq_doorbell_xa, flags);
 	}
 
 	if (found_hung_queue) {
+		drm_dev_wedged_event(adev_to_drm(adev), DRM_WEDGE_RECOVERY_NONE, NULL);
+
 		/* Resume scheduling after hang recovery */
 		r = amdgpu_mes_resume(adev, input.xcc_id);
 	}

-- 
2.51.2