Re: [Qemu-devel] [PATCH v2] nbd: fix the co_queue multi-adding bug

[Paolo Bonzini]

On 10/02/2015 08:20, Bin Wu wrote:
> From: Bin Wu <address@hidden>
> 
> 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 <address@hidden>
> ---
> v2: fix the coroutine re-enter bug in NBD code, not in coroutine 
> infrastructure
> as suggested by paolo and kevin.
> ---
>  block/nbd-client.c | 25 +++++++++++++------------
>  1 file changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/block/nbd-client.c b/block/nbd-client.c
> index 28bfb62..1b0f7c7 100644
> --- a/block/nbd-client.c
> +++ b/block/nbd-client.c
> @@ -104,10 +104,21 @@ static int nbd_co_send_request(NbdClientSession *s,
>      QEMUIOVector *qiov, int offset)
>  {
>      AioContext *aio_context;
> -    int rc, ret;
> +    int rc, ret, i;
>  
>      qemu_co_mutex_lock(&s->send_mutex);
> +
> +    for (i = 0; i < MAX_NBD_REQUESTS; i++) {
> +        if (s->recv_coroutine[i] == NULL) {
> +            s->recv_coroutine[i] = qemu_coroutine_self();
> +            break;
> +        }
> +    }
> +
> +    assert(i < MAX_NBD_REQUESTS);
> +    request->handle = INDEX_TO_HANDLE(s, i);
>      s->send_coroutine = qemu_coroutine_self();
> +
>      aio_context = bdrv_get_aio_context(s->bs);
>      aio_set_fd_handler(aio_context, s->sock,
>                         nbd_reply_ready, nbd_restart_write, s);
> @@ -164,8 +175,6 @@ static void nbd_co_receive_reply(NbdClientSession *s,
>  static void nbd_coroutine_start(NbdClientSession *s,
>     struct nbd_request *request)
>  {
> -    int i;
> -
>      /* Poor man semaphore.  The free_sema is locked when no other request
>       * can be accepted, and unlocked after receiving one reply.  */
>      if (s->in_flight >= MAX_NBD_REQUESTS - 1) {
> @@ -174,15 +183,7 @@ static void nbd_coroutine_start(NbdClientSession *s,
>      }
>      s->in_flight++;
>  
> -    for (i = 0; i < MAX_NBD_REQUESTS; i++) {
> -        if (s->recv_coroutine[i] == NULL) {
> -            s->recv_coroutine[i] = qemu_coroutine_self();
> -            break;
> -        }
> -    }
> -
> -    assert(i < MAX_NBD_REQUESTS);
> -    request->handle = INDEX_TO_HANDLE(s, i);
> +    /* s->recv_coroutine[i] is set as soon as we get the send_lock.  */
>  }
>  
>  static void nbd_coroutine_end(NbdClientSession *s,
> 

Signed-off-by: Paolo Bonzini <address@hidden>
Reviewed-by: Paolo Bonzini <address@hidden>