Re: [Qemu-devel] [PATCH for-2.1? 2/2] thread-pool: avoid deadlock in nested aio_poll() calls

[Paolo Bonzini]

Il 11/07/2014 13:20, Stefan Hajnoczi ha scritto:
> The thread pool has a race condition if two elements complete before
> thread_pool_completion_bh() runs:
>
>   If element A's callback waits for element B using aio_poll() it will
>   deadlock since pool->completion_bh is not marked scheduled when the
>   nested aio_poll() runs.
>
> Fix this by marking the BH scheduled while thread_pool_completion_bh()
> is executing.  This way any nested aio_poll() loops will enter
> thread_pool_completion_bh() and complete the remaining elements.
>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  thread-pool.c | 27 +++++++++++++++++++++++++--
>  1 file changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/thread-pool.c b/thread-pool.c
> index 4cfd078..0ede168 100644
> --- a/thread-pool.c
> +++ b/thread-pool.c
> @@ -65,6 +65,9 @@ struct ThreadPool {
>      int max_threads;
>      QEMUBH *new_thread_bh;
>
> +    /* Atomic counter to detect completions while completion handler runs */
> +    uint32_t completion_token;
> +
>      /* The following variables are only accessed from one AioContext. */
>      QLIST_HEAD(, ThreadPoolElement) head;
>
> @@ -118,6 +121,7 @@ static void *worker_thread(void *opaque)
>              qemu_cond_broadcast(&pool->check_cancel);
>          }
>
> +        atomic_inc(&pool->completion_token);
>          qemu_bh_schedule(pool->completion_bh);
>      }
>
> @@ -167,9 +171,8 @@ static void spawn_thread(ThreadPool *pool)
>      }
>  }
>
> -static void thread_pool_completion_bh(void *opaque)
> +static void thread_pool_complete_elements(ThreadPool *pool)
>  {
> -    ThreadPool *pool = opaque;
>      ThreadPoolElement *elem, *next;
>
>  restart:
> @@ -196,6 +199,26 @@ restart:
>      }
>  }
>
> +static void thread_pool_completion_bh(void *opaque)
> +{
> +    ThreadPool *pool = opaque;
> +    uint32_t token;
> +
> +    do {
> +        token = atomic_mb_read(&pool->completion_token);
> +
> +        /* Stay scheduled in case elem->common.cb() makes a nested aio_poll()
> +         * call.  This avoids deadlock if element A's callback waits for
> +         * element B and both completed at the same time.
> +         */
> +        qemu_bh_schedule(pool->completion_bh);
> +
> +        thread_pool_complete_elements(pool);
> +
> +        qemu_bh_cancel(pool->completion_bh);
> +    } while (token != pool->completion_token);
> +}
> +
>  static void thread_pool_cancel(BlockDriverAIOCB *acb)
>  {
>      ThreadPoolElement *elem = (ThreadPoolElement *)acb;

I am not sure I understand this patch.

The simplest way to fix deadlock is to change this in 
thread_pool_completion_bh:

            elem->common.cb(elem->common.opaque, elem->ret);
            qemu_aio_release(elem);
            goto restart;

to

            /* In case elem->common.cb() makes a nested aio_poll() call,
             * next may become invalid as well.  Instead of just
             * restarting the QLIST_FOREACH_SAFE, go through the BH
             * once more, which also avoids deadlock if element A's
             * callback waits for element B and both completed at the
             * same time.
             */
            qemu_bh_schedule(pool->completion_bh);
            elem->common.cb(elem->common.opaque, elem->ret);
            qemu_aio_release(elem);

There is no change in logic, it's just that the goto is switched to a BH 
representing a continuation.  I am then not sure why 
pool->completion_token is necessary?

Perhaps it is just an optimization to avoid going multiple times around 
aio_poll()?

Paolo