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

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH for-2.9] blockjob: avoid recursive AioContext locking
Date: Tue, 21 Mar 2017 14:15:43 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 03/21/2017 12:48 PM, 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

Why the blank line?

> 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(-)

Makes sense from the description.
Reviewed-by: Eric Blake <address@hidden>

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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