virtio-blk: On restart, process queued requests in the proper context

virtio-blk: On restart, process queued requests in the proper context

On restart, we were scheduling a BH to process queued requests, which
would run before starting up the data plane, leading to those requests
being assigned and started on coroutines on the main context.

This could cause requests to be wrongly processed in parallel from
different threads (the main thread and the iothread managing the data
plane), 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>
Message-Id: <20200603093240.40489-3-slp@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>