virtio-scsi: reset SCSI devices from main loop thread

When an IOThread is configured, the ctrl virtqueue is processed in the
IOThread. TMFs that reset SCSI devices are currently called directly
from the IOThread and trigger an assertion failure in blk_drain() from
the following call stack:

virtio_scsi_handle_ctrl_req -> virtio_scsi_do_tmf -> device_code_reset
-> scsi_disk_reset -> scsi_device_purge_requests -> blk_drain

  ../block/block-backend.c:1780: void blk_drain(BlockBackend *): Assertion `qemu_in_main_thread()' failed.

The blk_drain() function is not designed to be called from an IOThread
because it needs the Big QEMU Lock (BQL).

This patch defers TMFs that reset SCSI devices to a Bottom Half (BH)
that runs in the main loop thread under the BQL. This way it's safe to
call blk_drain() and the assertion failure is avoided.

Introduce s->tmf_bh_list for tracking TMF requests that have been
deferred to the BH. When the BH runs it will grab the entire list and
process all requests. Care must be taken to clear the list when the
virtio-scsi device is reset or unrealized. Otherwise deferred TMF
requests could execute later and lead to use-after-free or other
undefined behavior.

The s->resetting counter that's used by TMFs that reset SCSI devices is
accessed from multiple threads. This patch makes that explicit by using
atomic accessor functions. With this patch applied the counter is only
modified by the main loop thread under the BQL but can be read by any
thread.

Reported-by: Qing Wang <qinwang@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230221212218.1378734-4-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/include/hw/virtio/virtio-scsi.h b/include/hw/virtio/virtio-scsi.h
index 37b75e15e3..779568ab5d 100644
--- a/include/hw/virtio/virtio-scsi.h
+++ b/include/hw/virtio/virtio-scsi.h
@@ -74,13 +74,22 @@ struct VirtIOSCSICommon {
     VirtQueue **cmd_vqs;
 };
 
+struct VirtIOSCSIReq;
+
 struct VirtIOSCSI {
     VirtIOSCSICommon parent_obj;
 
     SCSIBus bus;
-    int resetting;
+    int resetting; /* written from main loop thread, read from any thread */
     bool events_dropped;
 
+    /*
+     * TMFs deferred to main loop BH. These fields are protected by
+     * virtio_scsi_acquire().
+     */
+    QEMUBH *tmf_bh;
+    QTAILQ_HEAD(, VirtIOSCSIReq) tmf_bh_list;
+
     /* Fields for dataplane below */
     AioContext *ctx; /* one iothread per virtio-scsi-pci for now */