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Re: [Qemu-devel] [PATCH v2 1/2] block: Forbid bdrv_set_aio_context outsi
From: |
Fam Zheng |
Subject: |
Re: [Qemu-devel] [PATCH v2 1/2] block: Forbid bdrv_set_aio_context outside BQL |
Date: |
Sun, 15 Feb 2015 11:05:17 +0800 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
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
>
>