Re: [Qemu-block] [Qemu-devel] [PATCH 3/3] migration: avoid recursive AioContext locking in save_vmstate()

[Eric Blake]

On 05/17/2017 12:09 PM, Stefan Hajnoczi wrote:
> AioContext was designed to allow nested acquire/release calls.  It uses
> a recursive mutex so callers don't need to worry about nesting...or so
> we thought.
> 
> BDRV_POLL_WHILE() is used to wait for block I/O requests.  It releases
> the AioContext temporarily around aio_poll().  This gives IOThreads a
> chance to acquire the AioContext to process I/O completions.
> 
> It turns out that recursive locking and BDRV_POLL_WHILE() don't mix.
> BDRV_POLL_WHILE() only releases the AioContext once, so the IOThread
> will not be able to acquire the AioContext if it was acquired
> multiple times.
> 
> Instead of trying to release AioContext n times in BDRV_POLL_WHILE(),
> this patch simply avoids nested locking in save_vmstate().  It's the
> simplest fix and we should step back to consider the big picture with
> all the recent changes to block layer threading.
> 
> This patch is the final fix to solve 'savevm' hanging with -object
> iothread.
> 
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  migration/savevm.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

signature.asc
Description: OpenPGP digital signature