Re: [Qemu-devel] [PATCH 13/16] block: only call aio_poll from iothread

[Fam Zheng <famz@redhat.com>]

On Tue, 02/16 18:56, Paolo Bonzini wrote:
> aio_poll is not thread safe; for example bdrv_drain can hang if
> the last in-flight I/O operation is completed in the I/O thread after
> the main thread has checked bs->in_flight.
> 
> The bug remains latent as long as all of it is called within
> aio_context_acquire/aio_context_release, but this will change soon.
> 
> To fix this, if bdrv_drain is called from outside the I/O thread handle
> it internally in the BDS, without involving AioContext and aio_poll.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  block.c                   |  2 ++
>  block/io.c                | 21 ++++++++++++++++++---
>  include/block/block_int.h |  5 ++++-
>  3 files changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/block.c b/block.c
> index fb02d7f..601a73f 100644
> --- a/block.c
> +++ b/block.c
> @@ -267,6 +267,7 @@ BlockDriverState *bdrv_new(void)
>      qemu_co_queue_init(&bs->throttled_reqs[1]);
>      bs->refcnt = 1;
>      bs->aio_context = qemu_get_aio_context();
> +    qemu_event_init(&bs->in_flight_event, true);
>  
>      QTAILQ_INSERT_TAIL(&all_bdrv_states, bs, bs_list);
>  
> @@ -2395,6 +2396,7 @@ static void bdrv_delete(BlockDriverState *bs)
>      bdrv_make_anon(bs);
>  
>      QTAILQ_REMOVE(&all_bdrv_states, bs, bs_list);
> +    qemu_event_destroy(&bs->in_flight_event);
>  
>      g_free(bs);
>  }
> diff --git a/block/io.c b/block/io.c
> index 04b52c8..ea0546f 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -251,11 +251,24 @@ static void bdrv_drain_recurse(BlockDriverState *bs)
>  
>  static bool bdrv_drain_io_recurse(BlockDriverState *bs)
>  {
> -    BdrvChild *child;
> +    AioContext *ctx = bdrv_get_aio_context(bs);
>      bool waited = false;
> +    BdrvChild *child;
>  
>      while (atomic_read(&bs->in_flight) > 0) {
> -        aio_poll(bdrv_get_aio_context(bs), true);
> +        if (aio_context_in_iothread(ctx)) {
> +            /* This case should not occur at all, except for the
> +             * main thread.
> +             */

Maybe assert ctx == qemu_get_aio_context()?

> +            aio_poll(bdrv_get_aio_context(bs), true);
> +        } else {
> +            qemu_event_reset(&bs->in_flight_event);
> +            if (atomic_read(&bs->in_flight) > 0) {
> +                aio_context_release(bdrv_get_aio_context(bs));
> +                qemu_event_wait(&bs->in_flight_event);
> +                aio_context_acquire(bdrv_get_aio_context(bs));
> +            }
> +        }
>          waited = true;
>      }
>  
> @@ -465,7 +478,9 @@ void bdrv_inc_in_flight(BlockDriverState *bs)
>  
>  void bdrv_dec_in_flight(BlockDriverState *bs)
>  {
> -    atomic_dec(&bs->in_flight);
> +    if (atomic_fetch_dec(&bs->in_flight) == 1) {
> +        qemu_event_set(&bs->in_flight_event);
> +    }
>  }
>  
>  static bool coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self)
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 89c38c0..9c96d5d 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -404,9 +404,12 @@ struct BlockDriverState {
>      /* Callback before write request is processed */
>      NotifierWithReturnList before_write_notifiers;
>  
> -    /* number of in-flight requests; overall and serialising */
> +    /* number of in-flight requests; overall and serialising.
> +     * in_flight_event is set when in_flight becomes 0.
> +     */
>      unsigned int in_flight;
>      unsigned int serialising_in_flight;
> +    QemuEvent in_flight_event;
>  
>      /* I/O throttling.
>       * throttle_state tells us if this BDS has I/O limits configured.
> -- 
> 2.5.0
> 
> 
>