Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer

[Kevin Wolf <kwolf@redhat.com>]

Am 12.05.2016 um 15:47 hat Alberto Garcia geschrieben:
> On Tue 03 May 2016 03:48:47 PM CEST, Kevin Wolf wrote:
> > Am 03.05.2016 um 15:33 hat Alberto Garcia geschrieben:
> >> On Tue 03 May 2016 03:23:24 PM CEST, Kevin Wolf wrote:
> >> >> c) we fix bdrv_reopen() so we can actually run both jobs at the same
> >> >>    time. I'm wondering if pausing all block jobs between
> >> >>    bdrv_reopen_prepare() and bdrv_reopen_commit() would do the
> >> >>    trick. Opinions?
> >> >
> >> > I would have to read up the details of the problem again, but I think
> >> > with bdrv_drained_begin/end() we actually have the right tool now to fix
> >> > it properly. We may need to pull up the drain (bdrv_drain_all() today)
> >> > from bdrv_reopen_multiple() to its caller and just assert it in the
> >> > function itself, but there shouldn't be much more to it than that.
> >> 
> >> I think that's not enough, see point 2) here:
> >> 
> >> https://lists.gnu.org/archive/html/qemu-block/2015-12/msg00180.html
> >> 
> >>   "I've been taking a look at the bdrv_drained_begin/end() API, but as I
> >>    understand it it prevents requests from a different AioContext.
> >>    Since all BDS in the same chain share the same context it does not
> >>    really help here."
> >
> > Yes, that's the part I meant with pulling up the calls.
> >
> > If I understand correctly, the problem is that first bdrv_reopen_queue()
> > queues a few BDSes for reopen, then bdrv_drain_all() completes all
> > running requests and can indirectly trigger a graph modification, and
> > then bdrv_reopen_multiple() uses the queue which doesn't match reality
> > any more.
> >
> > The solution to that should be simply changing the order of things:
> >
> > 1. bdrv_drained_begin()
> > 2. bdrv_reopen_queue()
> > 3. bdrv_reopen_multiple()
> >     * Instead of bdrv_drain_all(), assert that no requests are pending
> >     * We don't run requests, so we can't complete a block job and
> >       manipulate the graph any more
> > 4. then bdrv_drained_end()
> 
> This doesn't work. Here's what happens:
> 
> 1) Block job (a) starts (block-stream).
> 
> 2) Block job (b) starts (block-stream, or block-commit).
> 
> 3) job (b) calls bdrv_reopen() and does the drain call.
> 
> 4) job (b) creates reopen_queue and calls bdrv_reopen_multiple().
>    There are no pending requests at this point, but job (a) is sleeping.
> 
> 5) bdrv_reopen_multiple() iterates over reopen_queue and calls
>    bdrv_reopen_prepare() -> bdrv_flush() -> bdrv_co_flush() ->
>    qemu_coroutine_yield().

I think between here and the next step is what I don't understand.

bdrv_reopen_multiple() is not called in coroutine context, right? All
block jobs use block_job_defer_to_main_loop() before they call
bdrv_reopen(), as far as I can see. So bdrv_flush() shouldn't take the
shortcut, but use a nested event loop.

What is it that calls into job (a) from that event loop? It can't be a
request completion because we already drained all requests. Is it a
timer?

> 6) job (a) resumes, finishes the job and removes nodes from the graph.
> 
> 7) job (b) continues with bdrv_reopen_multiple() but now reopen_queue
>    contains invalid pointers.

I don't fully understand the problem yet, but as a shot in the dark,
would pausing block jobs in bdrv_drained_begin() help?

Kevin