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

[Kevin Wolf <kwolf@redhat.com>]

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.

Hmm, I may have unintentionally made this series obsolete with the drain
series I sent today. The poll instances that you're fixing simply don't
exist any more after it.

Can you check if the bug is gone with my series? I would hope so, but if
not, we would probably need to apply a fix in a different place.

Kevin