block/nbd.c: Fixed IO request coroutine not being wakeup when kill NBD server

During the IO stress test, the IO request coroutine has a probability that is
can't be awakened when the NBD server is killed.

The GDB stack is as follows:
(gdb) bt
0  0x00007f2ff990cbf6 in __ppoll (fds=0x55575de85000, nfds=1, timeout=<optimized out>, sigmask=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:44
1  0x000055575c302e7c in qemu_poll_ns (fds=0x55575de85000, nfds=1, timeout=599999603140) at ../util/qemu-timer.c:348
2  0x000055575c2d3c34 in fdmon_poll_wait (ctx=0x55575dc480f0, ready_list=0x7ffd9dd1dae0, timeout=599999603140) at ../util/fdmon-poll.c:80
3  0x000055575c2d350d in aio_poll (ctx=0x55575dc480f0, blocking=true) at ../util/aio-posix.c:655
4  0x000055575c16eabd in bdrv_do_drained_begin(bs=0x55575dee7fe0, recursive=false, parent=0x0, ignore_bds_parents=false, poll=true)at ../block/io.c:474
5  0x000055575c16eba6 in bdrv_drained_begin (bs=0x55575dee7fe0) at ../block/io.c:480
6  0x000055575c1aff33 in quorum_del_child (bs=0x55575dee7fe0, child=0x55575dcea690, errp=0x7ffd9dd1dd08) at ../block/quorum.c:1130
7  0x000055575c14239b in bdrv_del_child (parent_bs=0x55575dee7fe0, child=0x55575dcea690, errp=0x7ffd9dd1dd08) at ../block.c:7705
8  0x000055575c12da28 in qmp_x_blockdev_change(parent=0x55575df404c0 "colo-disk0", has_child=true, child=0x55575de867f0 "children.1", has_node=false, no   de=0x0, errp=0x7ffd9dd1dd08) at ../blockdev.c:3676
9  0x000055575c258435 in qmp_marshal_x_blockdev_change (args=0x7f2fec008190, ret=0x7f2ff7b0bd98, errp=0x7f2ff7b0bd90) at qapi/qapi-commands-block-core.c   :1675
10 0x000055575c2c6201 in do_qmp_dispatch_bh (opaque=0x7f2ff7b0be30) at ../qapi/qmp-dispatch.c:129
11 0x000055575c2ebb1c in aio_bh_call (bh=0x55575dc429c0) at ../util/async.c:141
12 0x000055575c2ebc2a in aio_bh_poll (ctx=0x55575dc480f0) at ../util/async.c:169
13 0x000055575c2d2d96 in aio_dispatch (ctx=0x55575dc480f0) at ../util/aio-posix.c:415
14 0x000055575c2ec07f in aio_ctx_dispatch (source=0x55575dc480f0, callback=0x0, user_data=0x0) at ../util/async.c:311
15 0x00007f2ff9e7cfbd in g_main_context_dispatch () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
16 0x000055575c2fd581 in glib_pollfds_poll () at ../util/main-loop.c:232
17 0x000055575c2fd5ff in os_host_main_loop_wait (timeout=0) at ../util/main-loop.c:255
18 0x000055575c2fd710 in main_loop_wait (nonblocking=0) at ../util/main-loop.c:531
19 0x000055575bfa7588 in qemu_main_loop () at ../softmmu/runstate.c:726
20 0x000055575bbee57a in main (argc=60, argv=0x7ffd9dd1e0e8, envp=0x7ffd9dd1e2d0) at ../softmmu/main.c:50

(gdb) qemu coroutine 0x55575e16aac0
0  0x000055575c2ee7dc in qemu_coroutine_switch (from_=0x55575e16aac0, to_=0x7f2ff830fba0, action=COROUTINE_YIELD) at ../util/coroutine-ucontext.c:302
1  0x000055575c2fe2a9 in qemu_coroutine_yield () at ../util/qemu-coroutine.c:195
2  0x000055575c2fe93c in qemu_co_queue_wait_impl (queue=0x55575dc46170, lock=0x7f2b32ad9850) at ../util/qemu-coroutine-lock.c:56
3  0x000055575c17ddfb in nbd_co_send_request (bs=0x55575ebfaf20, request=0x7f2b32ad9920, qiov=0x55575dfc15d8) at ../block/nbd.c:478
4  0x000055575c17f931 in nbd_co_request (bs=0x55575ebfaf20, request=0x7f2b32ad9920, write_qiov=0x55575dfc15d8) at ../block/nbd.c:1182
5  0x000055575c17fe14 in nbd_client_co_pwritev (bs=0x55575ebfaf20, offset=403487858688, bytes=4538368, qiov=0x55575dfc15d8, flags=0) at ../block/nbd.c:1284
6  0x000055575c170d25 in bdrv_driver_pwritev (bs=0x55575ebfaf20, offset=403487858688, bytes=4538368, qiov=0x55575dfc15d8, qiov_offset=0, flags=0)
    at ../block/io.c:1264
7  0x000055575c1733b4 in bdrv_aligned_pwritev
    (child=0x55575dff6890, req=0x7f2b32ad9ad0, offset=403487858688, bytes=4538368, align=1, qiov=0x55575dfc15d8, qiov_offset=0, flags=0) at ../block/io.c:2126
8  0x000055575c173c67 in bdrv_co_pwritev_part (child=0x55575dff6890, offset=403487858688, bytes=4538368, qiov=0x55575dfc15d8, qiov_offset=0, flags=0)
    at ../block/io.c:2314
9  0x000055575c17391b in bdrv_co_pwritev (child=0x55575dff6890, offset=403487858688, bytes=4538368, qiov=0x55575dfc15d8, flags=0) at ../block/io.c:2233
10 0x000055575c1ee506 in replication_co_writev (bs=0x55575e9824f0, sector_num=788062224, remaining_sectors=8864, qiov=0x55575dfc15d8, flags=0)
    at ../block/replication.c:270
11 0x000055575c170eed in bdrv_driver_pwritev (bs=0x55575e9824f0, offset=403487858688, bytes=4538368, qiov=0x55575dfc15d8, qiov_offset=0, flags=0)
    at ../block/io.c:1297
12 0x000055575c1733b4 in bdrv_aligned_pwritev
    (child=0x55575dcea690, req=0x7f2b32ad9e00, offset=403487858688, bytes=4538368, align=512, qiov=0x55575dfc15d8, qiov_offset=0, flags=0)
    at ../block/io.c:2126
13 0x000055575c173c67 in bdrv_co_pwritev_part (child=0x55575dcea690, offset=403487858688, bytes=4538368, qiov=0x55575dfc15d8, qiov_offset=0, flags=0)
    at ../block/io.c:2314
14 0x000055575c17391b in bdrv_co_pwritev (child=0x55575dcea690, offset=403487858688, bytes=4538368, qiov=0x55575dfc15d8, flags=0) at ../block/io.c:2233
15 0x000055575c1aeffa in write_quorum_entry (opaque=0x7f2fddaf8c50) at ../block/quorum.c:699
16 0x000055575c2ee4db in coroutine_trampoline (i0=1578543808, i1=21847) at ../util/coroutine-ucontext.c:173
17 0x00007f2ff9855660 in __start_context () at ../sysdeps/unix/sysv/linux/x86_64/__start_context.S:91

When we do failover in COLO mode, QEMU will hang while it is waiting for
the in-flight IO. From the call trace, we can see the IO request coroutine
has yielded in nbd_co_send_request(). When we kill the NBD server, it will never
be wake up. Actually, when we do IO stress test, it will have a lot of
requests in free_sema queue. When the NBD server is killed, current
MAX_NBD_REQUESTS finishes with errors but they wake up at most
MAX_NBD_REQEUSTS from the queue. So, let's move qemu_co_queue_next out
to fix this issue.

Signed-off-by: Lei Rao <lei.rao@intel.com>
Message-Id: <20220309074844.275450-1-lei.rao@intel.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

block/nbd.c

@@ -533,8 +533,8 @@ err:
         if (i != -1) {
             s->requests[i].coroutine = NULL;
             s->in_flight--;
-            qemu_co_queue_next(&s->free_sema);
         }
+        qemu_co_queue_next(&s->free_sema);
     }
     qemu_co_mutex_unlock(&s->send_mutex);
     return rc;