Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Kevin Wolf [mailto:kwolf@redhat.com]
> > >
> > > 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