Re: [Qemu-devel] [Qemu-block] [PATCH for-2.9] blockjob: avoid recursive AioContext locking

[Jeff Cody <jcody@redhat.com>]

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 <pbonzini@redhat.com>
> ---
>  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 <jcody@redhat.com>