Re: [Qemu-devel] [PATCH v2 1/2] block: Forbid bdrv_set_aio_context outside BQL

[Fam Zheng]

On Sun, 02/15 10:40, Fam Zheng wrote:
> Even if the caller has the old #AioContext, there can be a deadlock, due
> to the leading bdrv_drain_all:
> 
> Suppose there are three io threads (a, b, c) with each owning a BDS
> (bds_a, bds_b, bds_c), and a and b want to move their own BDS to c at
> the same time:
> 
>   iothread a                           iothread b
> --------------------------------------------------------------------------
>   bdrv_set_aio_context(bds_a, c)       bdrv_set_aio_context(bds_b, c)
>   -> bdrv_drain_all()                  -> bdrv_drain_all()
>      -> acquire a (OK, already has)       -> acquire a (blocked)
>      -> acquire b (blocked)               -> acquire b
>      -> acquire c                         -> acquire c

This doesn't recap the essence of the bug because one may argue that
aio_context_acquire(c) is needed before either thread calling
bdrv_set_aio_context. Actually it doesn't matter in this case, because iothread
b can as well do bdrv_set_aio_context(bds_b, d).

I'll update the commit log with v3.

Fam

> 
> Current caller of bdrv_set_aio_context outside BQL is
> virtio-scsi-dataplane, which will be fixed in the next patches.
> 
> Signed-off-by: Fam Zheng <address@hidden>
> Reviewed-by: Paolo Bonzini <address@hidden>
> ---
>  include/block/block.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/include/block/block.h b/include/block/block.h
> index 321295e..4fce25d 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -546,8 +546,7 @@ AioContext *bdrv_get_aio_context(BlockDriverState *bs);
>   * Changes the #AioContext used for fd handlers, timers, and BHs by this
>   * BlockDriverState and all its children.
>   *
> - * This function must be called from the old #AioContext or with a lock held 
> so
> - * the old #AioContext is not executing.
> + * This function must be called with iothread lock held.
>   */
>  void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context);
>  
> -- 
> 2.1.0
> 
>