Re: [PATCH v2 0/3] block: Start/end drain on correct AioContext

[Kevin Wolf <kwolf@redhat.com>]

Am 10.11.2022 um 17:11 hat Hanna Reitz geschrieben:
> On 10.11.22 15:01, Kevin Wolf wrote:
> > Am 07.11.2022 um 16:13 hat Hanna Reitz geschrieben:
> > > Hi,
> > > 
> > > v1 cover letter:
> > > https://lists.nongnu.org/archive/html/qemu-block/2022-09/msg00389.html
> > > 
> > > bdrv_replace_child_noperm() drains the child via
> > > bdrv_parent_drained_{begin,end}_single().  When it removes a child, the
> > > bdrv_parent_drained_end_single() at its end will be called on an empty
> > > child, making the BDRV_POLL_WHILE() in it poll the main AioContext
> > > (because c->bs is NULL).
> > > 
> > > That’s wrong, though, because it’s supposed to operate on the parent.
> > > bdrv_parent_drained_end_single_no_poll() will have scheduled any BHs in
> > > the parents’ AioContext, which may be anything, not necessarily the main
> > > context.  Therefore, we must poll the parent’s context.
> > > 
> > > Patch 3 does this for both bdrv_parent_drained_{begin,end}_single().
> > > Patch 1 ensures that we can legally call
> > > bdrv_child_get_parent_aio_context() from those I/O context functions,
> > > and patch 2 fixes blk_do_set_aio_context() to not cause an assertion
> > > failure if it beginning a drain can end up in blk_get_aio_context()
> > > before blk->ctx has been updated.
> > Thanks, applied to the block branch.
> 
> Thanks!
> 
> I tested your drain series, and it does indeed fix the bug, too.
> (Sorry for the delay, I thought it’d take less time to write an
> iotest...)

Thanks for testing, it's good to have this confirmed.

> > I would still be interested in a test case as a follow-up.
> 
> Got it working now and sent as “tests/stream-under-throttle: New test”.

Great! I'll take a look.

Kevin