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Re: [Qemu-block] [PATCH v2 2/4] block: Pause block jobs in bdrv_drain_al
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-block] [PATCH v2 2/4] block: Pause block jobs in bdrv_drain_all |
Date: |
Wed, 8 Apr 2015 11:37:52 +0100 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Fri, Apr 03, 2015 at 10:05:19PM +0800, Fam Zheng wrote:
> This is necessary to suppress more IO requests from being generated from
> block job coroutines.
>
> Signed-off-by: Fam Zheng <address@hidden>
> ---
> block.c | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/block.c b/block.c
> index f2f8ae7..00cd91e 100644
> --- a/block.c
> +++ b/block.c
> @@ -2033,6 +2033,16 @@ void bdrv_drain_all(void)
> bool busy = true;
> BlockDriverState *bs;
>
> + QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> + AioContext *aio_context = bdrv_get_aio_context(bs);
> +
> + aio_context_acquire(aio_context);
> + if (bs->job) {
> + block_job_pause(bs->job);
> + }
> + aio_context_release(aio_context);
> + }
> +
> while (busy) {
> busy = false;
>
> @@ -2044,6 +2054,16 @@ void bdrv_drain_all(void)
> aio_context_release(aio_context);
> }
> }
> +
> + QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> + AioContext *aio_context = bdrv_get_aio_context(bs);
> +
> + aio_context_acquire(aio_context);
> + if (bs->job) {
> + block_job_resume(bs->job);
> + }
> + aio_context_release(aio_context);
> + }
> }
There is a tiny chance that we pause a job (which actually just sets
job->paused = true but there's no guarantee the job's coroutine reacts
to this) right before it terminates. Then aio_poll() enters the
coroutine one last time and the job terminates.
We then reach the resume portion of bdrv_drain_all() and the job no
longer exists. Hopefully nothing started a new job in the meantime.
bs->job should either be the original block job or NULL.
This code seems under current assumptions, but I just wanted to raise
these issues in case someone sees problems that I've missed.
Stefan
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