> From: Pavel Dovgalyuk [mailto:dovga...@ispras.ru] > > From: Kevin Wolf [mailto:kw...@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.
Now I've encountered a situation where blkreplay_run_event is called from read coroutine: bdrv_prwv_co -> aio_poll -> qemu_clock_get_ns -> replay_read_clock -> blkreplay_run_event \--> bdrv_co_readv -> blkreplay_co_readv -> bdrv_co_readv(lower layer) bdrv_co_readv inside blkreplay_co_readv can't proceed in this situation. This is probably because aio_poll has taken the aio context? How can I resolve this? Pavel Dovgalyuk