Re: [Qemu-devel] [PATCH] nbd: Use CoQueue for free_sema instead of CoMutex

[Changlong Xie <xiecl.fnst@cn.fujitsu.com>]

Ping. Any comments? It's really a problem for NBD.

Thanks
	-Xie

On 10/12/2016 06:18 PM, Changlong Xie wrote:
> NBD is using the CoMutex in a way that wasn't anticipated. For example, if there are
> N(N=26, MAX_NBD_REQUESTS=16) nbd write requests, so we will invoke nbd_client_co_pwritev
> N times.
> ----------------------------------------------------------------------------------------
> time request Actions
> 1    1       in_flight=1, Coroutine=C1
> 2    2       in_flight=2, Coroutine=C2
> ...
> 15   15      in_flight=15, Coroutine=C15
> 16   16      in_flight=16, Coroutine=C16, free_sema->holder=C16, mutex->locked=true
> 17   17      in_flight=16, Coroutine=C17, queue C17 into free_sema->queue
> 18   18      in_flight=16, Coroutine=C18, queue C18 into free_sema->queue
> ...
> 26   N       in_flight=16, Coroutine=C26, queue C26 into free_sema->queue
> ----------------------------------------------------------------------------------------
>
> Once nbd client recieves request No.16' reply, we will re-enter C16. It's ok, because
> it's equal to 'free_sema->holder'.
> ----------------------------------------------------------------------------------------
> time request Actions
> 27   16      in_flight=15, Coroutine=C16, free_sema->holder=C16, mutex->locked=false
> ----------------------------------------------------------------------------------------
>
> Then nbd_coroutine_end invokes qemu_co_mutex_unlock what will pop coroutines from
> free_sema->queue's head and enter C17. More free_sema->holder is C17 now.
> ----------------------------------------------------------------------------------------
> time request Actions
> 28   17      in_flight=16, Coroutine=C17, free_sema->holder=C17, mutex->locked=true
> ----------------------------------------------------------------------------------------
>
> In above scenario, we only recieves request No.16' reply. As time goes by, nbd client will
> almostly recieves replies from requests 1 to 15 rather than request 17 who owns C17. In this
> case, we will encounter assert "mutex->holder == self" failed since Kevin's commit 0e438cdc
> "coroutine: Let CoMutex remember who holds it". For example, if nbd client recieves request
> No.15' reply, qemu will stop unexpectedly:
> ----------------------------------------------------------------------------------------
> time request       Actions
> 29   15(most case) in_flight=15, Coroutine=C15, free_sema->holder=C17, mutex->locked=false
> ----------------------------------------------------------------------------------------
>
> Per Paolo's suggestion "The simplest fix is to change it to CoQueue, which is like a condition
> variable", this patch replaces CoMutex with CoQueue.
>
> Cc: Wen Congyang <wency@cn.fujitsu.com>
> Reported-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
> ---
>   block/nbd-client.c | 8 ++++----
>   block/nbd-client.h | 2 +-
>   2 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/block/nbd-client.c b/block/nbd-client.c
> index 2cf3237..40b28ab 100644
> --- a/block/nbd-client.c
> +++ b/block/nbd-client.c
> @@ -199,8 +199,8 @@ static void nbd_coroutine_start(NbdClientSession *s,
>   {
>       /* 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) {
> -        qemu_co_mutex_lock(&s->free_sema);
> +    if (s->in_flight == MAX_NBD_REQUESTS) {
> +        qemu_co_queue_wait(&s->free_sema);
>           assert(s->in_flight < MAX_NBD_REQUESTS);
>       }
>       s->in_flight++;
> @@ -214,7 +214,7 @@ static void nbd_coroutine_end(NbdClientSession *s,
>       int i = HANDLE_TO_INDEX(s, request->handle);
>       s->recv_coroutine[i] = NULL;
>       if (s->in_flight-- == MAX_NBD_REQUESTS) {
> -        qemu_co_mutex_unlock(&s->free_sema);
> +        qemu_co_queue_next(&s->free_sema);
>       }
>   }
>
> @@ -386,7 +386,7 @@ int nbd_client_init(BlockDriverState *bs,
>       }
>
>       qemu_co_mutex_init(&client->send_mutex);
> -    qemu_co_mutex_init(&client->free_sema);
> +    qemu_co_queue_init(&client->free_sema);
>       client->sioc = sioc;
>       object_ref(OBJECT(client->sioc));
>
> diff --git a/block/nbd-client.h b/block/nbd-client.h
> index 044aca4..307b8b1 100644
> --- a/block/nbd-client.h
> +++ b/block/nbd-client.h
> @@ -24,7 +24,7 @@ typedef struct NbdClientSession {
>       off_t size;
>
>       CoMutex send_mutex;
> -    CoMutex free_sema;
> +    CoQueue free_sema;
>       Coroutine *send_coroutine;
>       int in_flight;
>
>