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Re: [Qemu-block] [PATCH for-2.9] blockjob: avoid recursive AioContext lo
From: |
Jeff Cody |
Subject: |
Re: [Qemu-block] [PATCH for-2.9] blockjob: avoid recursive AioContext locking |
Date: |
Wed, 22 Mar 2017 08:05:38 -0400 |
User-agent: |
Mutt/1.5.24 (2015-08-30) |
On Tue, Mar 21, 2017 at 06:48:10PM +0100, Paolo Bonzini wrote:
> Streaming or any other block job hangs when performed on a block device
> that has a non-default iothread. This happens because the AioContext
> is acquired twice by block_job_defer_to_main_loop_bh and then released
> only once by BDRV_POLL_WHILE. (Insert rants on recursive mutexes, which
>
> unfortunately are a temporary but necessary evil for iothreads at the
> moment).
>
> Luckily, the reason for the double acquisition is simple; the function
> acquires the AioContext for both the job iothread and the BDS iothread,
> in case the BDS iothread was changed while the job was running. It
> is therefore enough to skip the second acquisition when the two
> AioContexts are one and the same.
>
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
> blockjob.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/blockjob.c b/blockjob.c
> index 69126af..2159df7 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -755,12 +755,16 @@ static void block_job_defer_to_main_loop_bh(void
> *opaque)
>
> /* Fetch BDS AioContext again, in case it has changed */
> aio_context = blk_get_aio_context(data->job->blk);
> - aio_context_acquire(aio_context);
> + if (aio_context != data->aio_context) {
> + aio_context_acquire(aio_context);
> + }
>
> data->job->deferred_to_main_loop = false;
> data->fn(data->job, data->opaque);
>
> - aio_context_release(aio_context);
> + if (aio_context != data->aio_context) {
> + aio_context_release(aio_context);
> + }
>
> aio_context_release(data->aio_context);
>
> --
> 1.8.3.1
>
>
Reviewed-by: Jeff Cody <address@hidden>