nbd: fix the co_queue multi-adding bug

When we tested the VM migartion between different hosts with NBD
devices, we found if we sent a cancel command after the drive_mirror
was just started, a coroutine re-enter error would occur. The stack
was as follow:

(gdb) bt
00)  0x00007fdfc744d885 in raise () from /lib64/libc.so.6
01)  0x00007fdfc744ee61 in abort () from /lib64/libc.so.6
02)  0x00007fdfca467cc5 in qemu_coroutine_enter (co=0x7fdfcaedb400, opaque=0x0)
at qemu-coroutine.c:118
03)  0x00007fdfca467f6c in qemu_co_queue_run_restart (co=0x7fdfcaedb400) at
qemu-coroutine-lock.c:59
04)  0x00007fdfca467be5 in coroutine_swap (from=0x7fdfcaf3c4e8,
to=0x7fdfcaedb400) at qemu-coroutine.c:96
05)  0x00007fdfca467cea in qemu_coroutine_enter (co=0x7fdfcaedb400, opaque=0x0)
at qemu-coroutine.c:123
06)  0x00007fdfca467f6c in qemu_co_queue_run_restart (co=0x7fdfcaedbdc0) at
qemu-coroutine-lock.c:59
07)  0x00007fdfca467be5 in coroutine_swap (from=0x7fdfcaf3c4e8,
to=0x7fdfcaedbdc0) at qemu-coroutine.c:96
08)  0x00007fdfca467cea in qemu_coroutine_enter (co=0x7fdfcaedbdc0, opaque=0x0)
at qemu-coroutine.c:123
09)  0x00007fdfca4a1fa4 in nbd_recv_coroutines_enter_all (s=0x7fdfcaef7dd0) at
block/nbd-client.c:41
10) 0x00007fdfca4a1ff9 in nbd_teardown_connection (client=0x7fdfcaef7dd0) at
block/nbd-client.c:50
11) 0x00007fdfca4a20f0 in nbd_reply_ready (opaque=0x7fdfcaef7dd0) at
block/nbd-client.c:92
12) 0x00007fdfca45ed80 in aio_dispatch (ctx=0x7fdfcae15e90) at aio-posix.c:144
13) 0x00007fdfca45ef1b in aio_poll (ctx=0x7fdfcae15e90, blocking=false) at
aio-posix.c:222
14) 0x00007fdfca448c34 in aio_ctx_dispatch (source=0x7fdfcae15e90, callback=0x0,
user_data=0x0) at async.c:212
15) 0x00007fdfc8f2f69a in g_main_context_dispatch () from
/usr/lib64/libglib-2.0.so.0
16) 0x00007fdfca45c391 in glib_pollfds_poll () at main-loop.c:190
17) 0x00007fdfca45c489 in os_host_main_loop_wait (timeout=1483677098) at
main-loop.c:235
18) 0x00007fdfca45c57b in main_loop_wait (nonblocking=0) at main-loop.c:484
19) 0x00007fdfca25f403 in main_loop () at vl.c:2249
20) 0x00007fdfca266fc2 in main (argc=42, argv=0x7ffff517d638,
envp=0x7ffff517d790) at vl.c:4814

We find the nbd_recv_coroutines_enter_all function (triggered by a cancel
command or a network connection breaking down) will enter a coroutine which
is waiting for the sending lock. If the lock is still held by another coroutine,
the entering coroutine will be added into the co_queue again. Latter, when the
lock is released, a coroutine re-enter error will occur.

This bug can be fixed simply by delaying the setting of recv_coroutine as
suggested by paolo. After applying this patch, we have tested the cancel
operation in mirror phase looply for more than 5 hous and everything is fine.
Without this patch, a coroutine re-enter error will occur in 5 minutes.

Signed-off-by: Bn Wu <wu.wubin@huawei.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1423552846-3896-1-git-send-email-wu.wubin@huawei.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
