Re: [Qemu-devel] [PATCH 1/3] block: Forbid bdrv_set_aio_context outside BQL

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Feb 12, 2015 at 01:21:00PM +0800, 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
> 
> 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 <famz@redhat.com>
> ---
>  include/block/block.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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