[PATCH] vhost-scsi: do not relock event vq mutex on send_evt fallback

[Jia Jia <physicalmtea@gmail.com>]

vhost_scsi_send_evt() is called with the event virtqueue mutex held.
If the worker is gone, the fallback currently calls
vhost_scsi_complete_events(), which tries to acquire the same mutex again
and deadlocks the caller.

Split event completion into a helper for callers that already hold the
mutex and a locking wrapper for the event worker. Use the helper on the
fallback path.

Link: https://lore.kernel.org/all/20260905005352.1E5B01F00A3D@smtp.kernel.org/
Fixes: b1b2ce58ed23 ("vhost-scsi: Handle vhost_vq_work_queue failures for events")
Signed-off-by: Jia Jia <physicalmtea@gmail.com>
---
Sashiko AI flagged this while reviewing
the vhost-scsi event queue fix.
This is a pre-existing self-deadlock. It was reproduced in a follow-up test.

Trigger: vq->worker == NULL. vhost_vq_work_queue() then returns false,
and a subsequent vhost_scsi_do_plug() call deadlocks. I do not know what
normal condition gets us here; the normal vhost-scsi worker detach/reset
paths do not reach this code. The only reproduction I could come up with
was killing the vhost-scsi worker. This still looks like a low-probability
condition.

 drivers/vhost/scsi.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
index 7a1f39a32..7c905e9ad 100644
--- a/drivers/vhost/scsi.c
+++ b/drivers/vhost/scsi.c
@@ -630,19 +630,26 @@ vhost_scsi_do_evt_work(struct vhost_scsi *vs, struct vhost_scsi_evt *evt)
 	vhost_scsi_log_write(vq, vq_log, log_num);
 }
 
-static void vhost_scsi_complete_events(struct vhost_scsi *vs, bool drop)
+/* Caller must hold the event virtqueue mutex. */
+static void __vhost_scsi_complete_events(struct vhost_scsi *vs, bool drop)
 {
-	struct vhost_virtqueue *vq = &vs->vqs[VHOST_SCSI_VQ_EVT].vq;
 	struct vhost_scsi_evt *evt, *t;
 	struct llist_node *llnode;
 
-	mutex_lock(&vq->mutex);
 	llnode = llist_del_all(&vs->vs_event_list);
 	llist_for_each_entry_safe(evt, t, llnode, list) {
 		if (!drop)
 			vhost_scsi_do_evt_work(vs, evt);
 		vhost_scsi_free_evt(vs, evt);
 	}
+}
+
+static void vhost_scsi_complete_events(struct vhost_scsi *vs, bool drop)
+{
+	struct vhost_virtqueue *vq = &vs->vqs[VHOST_SCSI_VQ_EVT].vq;
+
+	mutex_lock(&vq->mutex);
+	__vhost_scsi_complete_events(vs, drop);
 	mutex_unlock(&vq->mutex);
 }
 
@@ -1829,7 +1836,7 @@ vhost_scsi_send_evt(struct vhost_scsi *vs, struct vhost_virtqueue *vq,
 
 	llist_add(&evt->list, &vs->vs_event_list);
 	if (!vhost_vq_work_queue(vq, &vs->vs_event_work))
-		vhost_scsi_complete_events(vs, true);
+		__vhost_scsi_complete_events(vs, true);
 }
 
 static void vhost_scsi_evt_handle_kick(struct vhost_work *work)