Re: [PATCH hci-8.0 1/1] hw/scsi: avoid deadlock upon TMF request cancelling with VirtIO #VSTOR-148264

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

Ack
Applied to hci-8.0

On 10/2/26 1:58 PM, Denis V. Lunev wrote:
> From: 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>
> Message-id: 20251017094518.328905-1-f.ebner@proxmox.com
> [Changed g_list_append() to g_list_prepend() to avoid traversing the
> list each time.
> --Stefan]
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> (cherry picked from commit 6910f04aa646f63a0257f77201ad8ea15992b816)
> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
> (cherry picked from commit dfd597681092ba72c2c050cd0aa1d80a87822804)
> ---
>  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 34ae14f7bf9..3b635053b56 100644
> --- a/hw/scsi/virtio-scsi.c
> +++ b/hw/scsi/virtio-scsi.c
> @@ -343,6 +343,7 @@ 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;
> @@ -378,10 +379,21 @@ 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_prepend(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);
>