Re: [Qemu-devel] [PATCH 06/14] block: Add missing locking in bdrv_co_drain_bh_cb()

[Fam Zheng <famz@redhat.com>]

On Fri, 09/07 18:15, Kevin Wolf wrote:
> bdrv_do_drained_begin/end() assume that they are called with the
> AioContext lock of bs held. If we call drain functions from a coroutine
> with the AioContext lock held, we yield and schedule a BH to move out of
> coroutine context. This means that the lock for the home context of the
> coroutine is released and must be re-acquired in the bottom half.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  include/qemu/coroutine.h |  5 +++++
>  block/io.c               | 15 +++++++++++++++
>  util/qemu-coroutine.c    |  5 +++++
>  3 files changed, 25 insertions(+)
> 
> diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
> index 6f8a487041..9801e7f5a4 100644
> --- a/include/qemu/coroutine.h
> +++ b/include/qemu/coroutine.h
> @@ -90,6 +90,11 @@ void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co);
>  void coroutine_fn qemu_coroutine_yield(void);
>  
>  /**
> + * Get the AioContext of the given coroutine
> + */
> +AioContext *coroutine_fn qemu_coroutine_get_aio_context(Coroutine *co);
> +
> +/**
>   * Get the currently executing coroutine
>   */
>  Coroutine *coroutine_fn qemu_coroutine_self(void);
> diff --git a/block/io.c b/block/io.c
> index 7100344c7b..914ba78f1a 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -288,6 +288,18 @@ static void bdrv_co_drain_bh_cb(void *opaque)
>      BlockDriverState *bs = data->bs;
>  
>      if (bs) {
> +        AioContext *ctx = bdrv_get_aio_context(bs);
> +        AioContext *co_ctx = qemu_coroutine_get_aio_context(co);
> +
> +        /*
> +         * When the coroutine yielded, the lock for its home context was
> +         * released, so we need to re-acquire it here. If it explicitly
> +         * acquired a different context, the lock is still held and we don't
> +         * want to lock it a second time (or AIO_WAIT_WHILE() would hang).
> +         */

This condition is rather obscure. When is ctx not equal to co_ctx?

> +        if (ctx == co_ctx) {
> +            aio_context_acquire(ctx);
> +        }
>          bdrv_dec_in_flight(bs);
>          if (data->begin) {
>              bdrv_do_drained_begin(bs, data->recursive, data->parent,
> @@ -296,6 +308,9 @@ static void bdrv_co_drain_bh_cb(void *opaque)
>              bdrv_do_drained_end(bs, data->recursive, data->parent,
>                                  data->ignore_bds_parents);
>          }
> +        if (ctx == co_ctx) {
> +            aio_context_release(ctx);
> +        }
>      } else {
>          assert(data->begin);
>          bdrv_drain_all_begin();
> diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> index 1ba4191b84..2295928d33 100644
> --- a/util/qemu-coroutine.c
> +++ b/util/qemu-coroutine.c
> @@ -198,3 +198,8 @@ bool qemu_coroutine_entered(Coroutine *co)
>  {
>      return co->caller;
>  }
> +
> +AioContext *coroutine_fn qemu_coroutine_get_aio_context(Coroutine *co)
> +{
> +    return co->ctx;
> +}
> -- 
> 2.13.6
>