[PATCH] virtio-blk: Disable request queuing while switching contexts

[Sergio Lopez <slp@redhat.com>]

Disable request queuing while switching contexts on
virtio_blk_data_plane_[start|stop](), preventing requests from getting
queued on the wrong context.

Placing requests on the wrong context may lead to them being wrongly
accessed in parallel from different threads, potentially leading to
multiple issues.

For example, stopping and resuming a VM multiple times while the guest
is generating I/O on a virtio_blk device can trigger a crash with a
stack tracing looking like this one:

<------>
 Thread 2 (Thread 0x7ff736765700 (LWP 1062503)):
 #0  0x00005567a13b99d6 in iov_memset
     (iov=0x6563617073206f4e, iov_cnt=1717922848, offset=516096, fillc=0, bytes=7018105756081554803)
     at util/iov.c:69
 #1  0x00005567a13bab73 in qemu_iovec_memset
     (qiov=0x7ff73ec99748, offset=516096, fillc=0, bytes=7018105756081554803) at util/iov.c:530
 #2  0x00005567a12f411c in qemu_laio_process_completion (laiocb=0x7ff6512ee6c0) at block/linux-aio.c:86
 #3  0x00005567a12f42ff in qemu_laio_process_completions (s=0x7ff7182e8420) at block/linux-aio.c:217
 #4  0x00005567a12f480d in ioq_submit (s=0x7ff7182e8420) at block/linux-aio.c:323
 #5  0x00005567a12f43d9 in qemu_laio_process_completions_and_submit (s=0x7ff7182e8420)
     at block/linux-aio.c:236
 #6  0x00005567a12f44c2 in qemu_laio_poll_cb (opaque=0x7ff7182e8430) at block/linux-aio.c:267
 #7  0x00005567a13aed83 in run_poll_handlers_once (ctx=0x5567a2b58c70, timeout=0x7ff7367645f8)
     at util/aio-posix.c:520
 #8  0x00005567a13aee9f in run_poll_handlers (ctx=0x5567a2b58c70, max_ns=16000, timeout=0x7ff7367645f8)
     at util/aio-posix.c:562
 #9  0x00005567a13aefde in try_poll_mode (ctx=0x5567a2b58c70, timeout=0x7ff7367645f8)
     at util/aio-posix.c:597
 #10 0x00005567a13af115 in aio_poll (ctx=0x5567a2b58c70, blocking=true) at util/aio-posix.c:639
 #11 0x00005567a109acca in iothread_run (opaque=0x5567a2b29760) at iothread.c:75
 #12 0x00005567a13b2790 in qemu_thread_start (args=0x5567a2b694c0) at util/qemu-thread-posix.c:519
 #13 0x00007ff73eedf2de in start_thread () at /lib64/libpthread.so.0
 #14 0x00007ff73ec10e83 in clone () at /lib64/libc.so.6

 Thread 1 (Thread 0x7ff743986f00 (LWP 1062500)):
 #0  0x00005567a13b99d6 in iov_memset
     (iov=0x6563617073206f4e, iov_cnt=1717922848, offset=516096, fillc=0, bytes=7018105756081554803)
     at util/iov.c:69
 #1  0x00005567a13bab73 in qemu_iovec_memset
     (qiov=0x7ff73ec99748, offset=516096, fillc=0, bytes=7018105756081554803) at util/iov.c:530
 #2  0x00005567a12f411c in qemu_laio_process_completion (laiocb=0x7ff6512ee6c0) at block/linux-aio.c:86
 #3  0x00005567a12f42ff in qemu_laio_process_completions (s=0x7ff7182e8420) at block/linux-aio.c:217
 #4  0x00005567a12f480d in ioq_submit (s=0x7ff7182e8420) at block/linux-aio.c:323
 #5  0x00005567a12f4a2f in laio_do_submit (fd=19, laiocb=0x7ff5f4ff9ae0, offset=472363008, type=2)
     at block/linux-aio.c:375
 #6  0x00005567a12f4af2 in laio_co_submit
     (bs=0x5567a2b8c460, s=0x7ff7182e8420, fd=19, offset=472363008, qiov=0x7ff5f4ff9ca0, type=2)
     at block/linux-aio.c:394
 #7  0x00005567a12f1803 in raw_co_prw
     (bs=0x5567a2b8c460, offset=472363008, bytes=20480, qiov=0x7ff5f4ff9ca0, type=2)
     at block/file-posix.c:1892
 #8  0x00005567a12f1941 in raw_co_pwritev
     (bs=0x5567a2b8c460, offset=472363008, bytes=20480, qiov=0x7ff5f4ff9ca0, flags=0)
     at block/file-posix.c:1925
 #9  0x00005567a12fe3e1 in bdrv_driver_pwritev
     (bs=0x5567a2b8c460, offset=472363008, bytes=20480, qiov=0x7ff5f4ff9ca0, qiov_offset=0, flags=0)
     at block/io.c:1183
 #10 0x00005567a1300340 in bdrv_aligned_pwritev
     (child=0x5567a2b5b070, req=0x7ff5f4ff9db0, offset=472363008, bytes=20480, align=512, qiov=0x7ff72c0425b8, qiov_offset=0, flags=0) at block/io.c:1980
 #11 0x00005567a1300b29 in bdrv_co_pwritev_part
     (child=0x5567a2b5b070, offset=472363008, bytes=20480, qiov=0x7ff72c0425b8, qiov_offset=0, flags=0)
     at block/io.c:2137
 #12 0x00005567a12baba1 in qcow2_co_pwritev_task
     (bs=0x5567a2b92740, file_cluster_offset=472317952, offset=487305216, bytes=20480, qiov=0x7ff72c0425b8, qiov_offset=0, l2meta=0x0) at block/qcow2.c:2444
 #13 0x00005567a12bacdb in qcow2_co_pwritev_task_entry (task=0x5567a2b48540) at block/qcow2.c:2475
 #14 0x00005567a13167d8 in aio_task_co (opaque=0x5567a2b48540) at block/aio_task.c:45
 #15 0x00005567a13cf00c in coroutine_trampoline (i0=738245600, i1=32759) at util/coroutine-ucontext.c:115
 #16 0x00007ff73eb622e0 in __start_context () at /lib64/libc.so.6
 #17 0x00007ff6626f1350 in  ()
 #18 0x0000000000000000 in  ()
<------>

This is also known to cause crashes with this message (assertion
failed):

 aio_co_schedule: Co-routine was already scheduled in 'aio_co_schedule'

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1812765
Signed-off-by: Sergio Lopez <slp@redhat.com>
---
 hw/block/dataplane/virtio-blk.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 1b52e8159c..f1c7ba69c0 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -214,12 +214,17 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
     vblk->dataplane_started = true;
     trace_virtio_blk_data_plane_start(s);
 
+    /* Prevent requests from getting queued on the old context */
+    blk_set_disable_request_queuing(s->conf->conf.blk, true);
+
     r = blk_set_aio_context(s->conf->conf.blk, s->ctx, &local_err);
     if (r < 0) {
         error_report_err(local_err);
         goto fail_guest_notifiers;
     }
 
+    blk_set_disable_request_queuing(s->conf->conf.blk, false);
+
     /* Kick right away to begin processing requests already in vring */
     for (i = 0; i < nvqs; i++) {
         VirtQueue *vq = virtio_get_queue(s->vdev, i);
@@ -287,10 +292,15 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
     aio_context_acquire(s->ctx);
     aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s);
 
+    /* Prevent requests from getting queued on the old context */
+    blk_set_disable_request_queuing(s->conf->conf.blk, true);
+
     /* Drain and try to switch bs back to the QEMU main loop. If other users
      * keep the BlockBackend in the iothread, that's ok */
     blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context(), NULL);
 
+    blk_set_disable_request_queuing(s->conf->conf.blk, false);
+
     aio_context_release(s->ctx);
 
     for (i = 0; i < nvqs; i++) {
-- 
2.26.2