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Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/repl
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay |
Date: |
Mon, 15 Feb 2016 10:10:16 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 15.02.2016 um 09:38 hat Pavel Dovgalyuk geschrieben:
> > From: Kevin Wolf [mailto:address@hidden
> > > >
> > > > int blkreplay_co_readv()
> > > > {
> > > > BlockReplayState *s = bs->opaque;
> > > > int reqid = s->reqid++;
> > > >
> > > > bdrv_co_readv(bs->file, ...);
> > > >
> > > > if (mode == record) {
> > > > log(reqid, time);
> > > > } else {
> > > > assert(mode == replay);
> > > > bool *done = req_replayed_list_get(reqid)
> > > > if (done) {
> > > > *done = true;
> > > > } else {
> > > point A
> > > > req_completed_list_insert(reqid, qemu_coroutine_self());
> > > > qemu_coroutine_yield();
> > > > }
> > > > }
> > > > }
> > > >
> > > > /* called by replay.c */
> > > > int blkreplay_run_event()
> > > > {
> > > > if (mode == replay) {
> > > > co = req_completed_list_get(e.reqid);
> > > > if (co) {
> > > > qemu_coroutine_enter(co);
> > > > } else {
> > > > bool done = false;
> > > > req_replayed_list_insert(reqid, &done);
> > > point B
> > > > /* wait synchronously for completion */
> > > > while (!done) {
> > > > aio_poll();
> > > > }
> > > > }
> > > > }
> > > > }
> > >
> > > One more question about coroutines.
> > > Are race conditions possible in this sample?
> > > In replay mode we may call readv, and reach point A.
> > > On the same time, we will read point B in another thread.
> > > Then readv will yield and nobody will start it back?
> >
> > There are two aspects to this:
> >
> > * Real multithreading doesn't exist in the block layer. All block driver
> > functions are only called with the mutex in the AioContext held. There
> > is exactly one AioContext per BDS, so no two threads can possible be
> > operating on the same BDS at the same time.
> >
> > * Coroutines are different from threads in that they aren't preemptive.
> > They are only interrupted in places where they explicitly yield.
> >
> > Of course, in order for this to work, we actually need to take the mutex
> > before calling blkreplay_run_event(), which is called directly from the
> > replay code (which runs in the mainloop thread? Or vcpu?).
>
> blkreplay_run_event() is called from replay code which is protected by mutex.
> This function may be called from io and vcpu threads, because both of them
> have replay functions invocations.
I assume that the mutex you mean is the BQL. This is enough for cases
which use the main loop AioContext, but not for block devices with
dataplane threads.
> > So I think you need to have a aio_context_acquire(bs->aio_context) and
> > aio_context_release(bs->aio_context) around the function; either here or
> > in the calling replay code.
>
> And what about coroutine code? Does it call aio_context_acquire somewhere?
The coroutine code doesn't call aio_context_acquire() by itself. But
coroutines may only be entered when the appropriate AioContext lock is
held, so they are always protected.
Kevin
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, (continued)
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/11
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Kevin Wolf, 2016/02/11
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/11
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Kevin Wolf, 2016/02/11
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/11
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/12
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Kevin Wolf, 2016/02/12
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/12
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Kevin Wolf, 2016/02/12
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/15
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay,
Kevin Wolf <=
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/15
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Kevin Wolf, 2016/02/15
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/15
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Kevin Wolf, 2016/02/15
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/15
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Kevin Wolf, 2016/02/15
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/15
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Kevin Wolf, 2016/02/15
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Pavel Dovgalyuk, 2016/02/16
- Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay, Kevin Wolf, 2016/02/16