> 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. > 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
