Re: [Qemu-devel] [PATCH 07/11] block: let backup blockjob run in BDS AioContext

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Oct 01, 2014 at 09:38:39PM +0200, Max Reitz wrote:
> On 01.10.2014 19:01, Stefan Hajnoczi wrote:
> >The backup block job must run in the BlockDriverState AioContext so that
> >it works with dataplane.
> >
> >The basics of acquiring the AioContext are easy in blockdev.c.
> >
> >The completion code in block/backup.c must call bdrv_unref() from the
> >main loop.  Use block_job_defer_to_main_loop() to achieve that.
> >
> >Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> >---
> >  block/backup.c | 21 +++++++++++++++++++--
> >  blockdev.c     | 23 ++++++++++++++++-------
> >  2 files changed, 35 insertions(+), 9 deletions(-)
> 
> Hm, so, if I run a backup blockjob from one thread (which is not the main
> loop and not the BDS's AIO context)

That cannot happen.  The blockjob always runs with the BDS AioContext
acquired - either because we explicitly did so in the main loop or
because the dataplane IOThread acquires it.

> and try to cancel it or set its speed
> from another thread (e.g. the main loop), won't the blockjob hold the BDS's
> AIO context lock as long as it runs and making it impossible to interfere?

No, because of how the AioContext's RfifoLock lock works.

It has a "contention callback" that kicks the aio_poll() loop.  This
allows the QEMU monitor to acquire the lock whenever an IOThread is
blocked in poll(2).  It uses an eventfd to force the IOThread out of
poll(2) and this happens automatically in aio_context_acquire().

See aio_context_new() and aio_rfifolock_cb().

Stefan

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