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Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/repl
From: |
Pavel Dovgalyuk |
Subject: |
Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay |
Date: |
Mon, 15 Feb 2016 11:38:53 +0300 |
> 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.
> 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?
Pavel Dovgalyuk
- 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, Kevin Wolf, 2016/02/10
- 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 <=
- 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, 2016/02/15
- 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