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Re: [Qemu-devel] [PATCH 2/3] block: use BDRV_POLL_WHILE() in bdrv_rw_vms
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH 2/3] block: use BDRV_POLL_WHILE() in bdrv_rw_vmstate() |
Date: |
Wed, 17 May 2017 15:16:08 -0500 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0 |
On 05/17/2017 12:09 PM, Stefan Hajnoczi wrote:
> Calling aio_poll() directly may have been fine previously, but this is
> the future, man!
lol
> The difference between an aio_poll() loop and
> BDRV_POLL_WHILE() is that BDRV_POLL_WHILE() releases the AioContext
> around aio_poll().
>
> This allows the IOThread to run fd handlers or BHs to complete the
> request. Failure to release the AioContext causes deadlocks.
>
> Using BDRV_POLL_WHILE() partially fixes a 'savevm' hang with -object
> iothread.
I'm surprised at how many separate hangs we actually had!
>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
> block/io.c | 4 +---
> 1 file changed, 1 insertion(+), 3 deletions(-)
Reviewed-by: Eric Blake <address@hidden>
>
> diff --git a/block/io.c b/block/io.c
> index cc56e90..f0041cd 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -2031,9 +2031,7 @@ bdrv_rw_vmstate(BlockDriverState *bs, QEMUIOVector
> *qiov, int64_t pos,
> Coroutine *co = qemu_coroutine_create(bdrv_co_rw_vmstate_entry,
> &data);
>
> bdrv_coroutine_enter(bs, co);
> - while (data.ret == -EINPROGRESS) {
> - aio_poll(bdrv_get_aio_context(bs), true);
> - }
> + BDRV_POLL_WHILE(bs, data.ret == -EINPROGRESS);
> return data.ret;
> }
Do we have other culprits (not necessarily for vmsave, but for other
situations), where we should be using BDRV_POLL_WHILE in separate
patches? For example, a quick grep show that at least hw/9pfs/9p.c makes
a direct call to aio_poll(,true) in a while loop.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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[Qemu-devel] [PATCH 3/3] migration: avoid recursive AioContext locking in save_vmstate(), Stefan Hajnoczi, 2017/05/17
Re: [Qemu-devel] [PATCH 0/3] block: fix 'savevm' hang with -object iothread, Paolo Bonzini, 2017/05/17