[PATCH v2] hw/scsi: avoid deadlock upon TMF request cancelling with VirtIO

[Fiona Ebner <f.ebner@proxmox.com>]

When scsi_req_dequeue() is reached via
scsi_req_cancel_async()
virtio_scsi_tmf_cancel_req()
virtio_scsi_do_tmf_aio_context(),
there is a deadlock when trying to acquire the SCSI device's requests
lock, because it was already acquired in
virtio_scsi_do_tmf_aio_context().

In particular, the issue happens with a FreeBSD guest (13, 14, 15,
maybe more), when it cancels SCSI requests, because of timeout.

This is a regression caused by commit da6eebb33b ("virtio-scsi:
perform TMFs in appropriate AioContexts") and the introduction of the
requests_lock earlier.

To fix the issue, only cancel the requests after releasing the
requests_lock. For this, the SCSI device's requests are iterated while
holding the requests_lock and the requests to be cancelled are
collected in a list. Then, the collected requests are cancelled
one by one while not holding the requests_lock. This is safe, because
only requests from the current AioContext are collected and acted
upon.

Originally reported by Proxmox VE users:
https://bugzilla.proxmox.com/show_bug.cgi?id=6810
https://forum.proxmox.com/threads/173914/

Fixes: da6eebb33b ("virtio-scsi: perform TMFs in appropriate AioContexts")
Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

Changes in v2:
* Different approach, collect requests for cancelling in a list for a
  localized solution rather than keeping track of the lock status via
  function arguments.

 hw/scsi/virtio-scsi.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index d817fc42b4..2896b05808 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -339,6 +339,7 @@ static void virtio_scsi_do_tmf_aio_context(void *opaque)
     SCSIDevice *d = virtio_scsi_device_get(s, tmf->req.tmf.lun);
     SCSIRequest *r;
     bool match_tag;
+    g_autoptr(GList) reqs = NULL;
 
     if (!d) {
         tmf->resp.tmf.response = VIRTIO_SCSI_S_BAD_TARGET;
@@ -374,10 +375,21 @@ static void virtio_scsi_do_tmf_aio_context(void *opaque)
             if (match_tag && cmd_req->req.cmd.tag != tmf->req.tmf.tag) {
                 continue;
             }
-            virtio_scsi_tmf_cancel_req(tmf, r);
+            /*
+             * Cannot cancel directly, because scsi_req_dequeue() would deadlock
+             * when attempting to acquire the request_lock a second time. Taking
+             * a reference here is paired with an unref after cancelling below.
+             */
+            scsi_req_ref(r);
+            reqs = g_list_append(reqs, r);
         }
     }
 
+    for (GList *elem = g_list_first(reqs); elem; elem = g_list_next(elem)) {
+        virtio_scsi_tmf_cancel_req(tmf, elem->data);
+        scsi_req_unref(elem->data);
+    }
+
     /* Incremented by virtio_scsi_do_tmf() */
     virtio_scsi_tmf_dec_remaining(tmf);
 
-- 
2.47.3