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Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming t
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer |
Date: |
Thu, 12 May 2016 17:04:51 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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
- Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer, Alberto Garcia, 2016/05/03
- Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer, Kevin Wolf, 2016/05/03
- Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer, Alberto Garcia, 2016/05/03
- Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer, Kevin Wolf, 2016/05/03
- Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer, Alberto Garcia, 2016/05/03
- Message not available
- Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer,
Kevin Wolf <=
- Message not available
- Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer, Kevin Wolf, 2016/05/12
- Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer, Alberto Garcia, 2016/05/17
- Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer, Kevin Wolf, 2016/05/17
- Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer, Alberto Garcia, 2016/05/17