Re: [Qemu-block] [PATCH for-4.1 2/2] block: Only the main loop can change AioContexts

[Kevin Wolf]

Am 23.07.2019 um 12:21 hat Max Reitz geschrieben:
> On 23.07.19 12:02, Kevin Wolf wrote:
> > Am 23.07.2019 um 11:41 hat Max Reitz geschrieben:
> >> On 23.07.19 10:52, Kevin Wolf wrote:
> >>> Am 22.07.2019 um 15:30 hat Max Reitz geschrieben:
> >>>> bdrv_set_aio_context_ignore() can only work in the main loop:
> >>>> bdrv_drained_begin() only works in the main loop and the node's (old)
> >>>> AioContext; and bdrv_drained_end() really only works in the main loop
> >>>> and the node's (new) AioContext (contrary to its current comment, which
> >>>> is just wrong).
> >>>>
> >>>> Consequentially, bdrv_set_aio_context_ignore() must be called from the
> >>>> main loop.  Luckily, assuming that we can make block graph changes only
> >>>> from the main loop as well, all its callers do that already.
> >>>>
> >>>> Note that changing a node's context in a sense is an operation that
> >>>> changes the block graph, so it actually makes sense to require this
> >>>> function to be called from the main loop.
> >>>>
> >>>> Also, fix bdrv_drained_end()'s description.  You can only use it from
> >>>> the main loop or the node's AioContext, and in the latter case, the
> >>>> whole subtree must be in the same context.
> >>>>
> >>>> Fixes: e037c09c78520cbdb6da7cfc6ad0256d5870b814
> >>>> Signed-off-by: Max Reitz <address@hidden>
> >>>> ---
> >>>>  include/block/block.h |  8 +++-----
> >>>>  block.c               | 13 ++++++++-----
> >>>>  2 files changed, 11 insertions(+), 10 deletions(-)
> >>>>
> >>>> diff --git a/include/block/block.h b/include/block/block.h
> >>>> index 60f00479e0..50a07c1c33 100644
> >>>> --- a/include/block/block.h
> >>>> +++ b/include/block/block.h
> >>>> @@ -667,11 +667,9 @@ void bdrv_subtree_drained_begin(BlockDriverState 
> >>>> *bs);
> >>>>   *
> >>>>   * This polls @bs's AioContext until all scheduled sub-drained_ends
> >>>>   * have settled.  On one hand, that may result in graph changes.  On
> >>>> - * the other, this requires that all involved nodes (@bs and all of
> >>>> - * its parents) are in the same AioContext, and that the caller has
> >>>> - * acquired it.
> >>>> - * If there are any nodes that are in different contexts from @bs,
> >>>> - * these contexts must not be acquired.
> >>>> + * the other, this requires that the caller either runs in the main
> >>>> + * loop; or that all involved nodes (@bs and all of its parents) are
> >>>> + * in the caller's AioContext.
> >>>>   */
> >>>>  void bdrv_drained_end(BlockDriverState *bs);
> >>>
> >>> I think you are right about the requirement that bdrv_drained_end() is
> >>> only called from the main or the BDS AioContext, which is a requirement
> >>> that directly comes from AIO_WAIT_WHILE().
> >>>
> >>> However, I don't see why we have requirements on the AioContext of the
> >>> parent nodes (or any other nodes), except possibly not holding their
> >>> lock.  We don't poll their context, so it shouldn't matter in which
> >>> context they are?
> >>
> >> Hm.  I don’t know how I got confused there, you’re right.
> >>
> >> I don’t think the (falsely given) restriction hurts, though, because a
> >> subtree should be within a single context anyway (unless you’re in
> >> bdrv_set_aio_context_ignore(), which needs to be in the main context).
> >>
> >> So, hm, yes, I messed up this comment a bit now.  But now it’s just more
> >> restrictive than it needs to be and I don’t think callers are going to
> >> care, so...
> > 
> > Nothing that should hold up your pull request, but I'd prefer to fix the
> > comment in a follow-up.
> 
> On second thought, does aio_wait_kick() wake up any context but the main
> context?  I was under the impression that it doesn’t.  If it doesn’t, I
> don’t know how bdrv_drained_end()’s AIO_WAIT_WHILE() will be woken up if
> it doesn’t run in the main context and it has to wait for yet another
> thread.

Hm, I think you're right.

And your comment actually says main loop _or_ everything in the same
context, so it's correct (but I misread it at first and thought the
condition would always apply).

Kevin

signature.asc
Description: PGP signature