From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60428) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aVFEi-0006WJ-LU for qemu-devel@nongnu.org; Mon, 15 Feb 2016 04:14:29 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aVFEf-0003ga-Dd for qemu-devel@nongnu.org; Mon, 15 Feb 2016 04:14:28 -0500 Received: from mail.ispras.ru ([83.149.199.45]:48742) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aVFEf-0003gW-18 for qemu-devel@nongnu.org; Mon, 15 Feb 2016 04:14:25 -0500 From: "Pavel Dovgalyuk" References: <20160209055506.8208.67.stgit@PASHA-ISP> <20160209055524.8208.16023.stgit@PASHA-ISP> <20160209102739.GB8554@noname.redhat.com> <001201d16330$6ce67e40$46b37ac0$@Dovgaluk@ispras.ru> <20160210114531.GA5474@noname.redhat.com> <000601d163fb$4cbcae70$e6360b50$@Dovgaluk@ispras.ru> <20160210122816.GB5474@noname.redhat.com> <000a01d16401$be4d31d0$3ae79570$@Dovgaluk@ispras.ru> <20160210132545.GC5474@noname.redhat.com> <001201d16597$fa5de6a0$ef19b3e0$@Dovgaluk@ispras.ru> <20160212135820.GD4828@noname.redhat.com> <003301d167cc$4d7d9480$e878bd80$@ru> In-Reply-To: <003301d167cc$4d7d9480$e878bd80$@ru> Date: Mon, 15 Feb 2016 12:14:23 +0300 Message-ID: <003a01d167d1$42df95f0$c89ec1d0$@ru> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Content-Language: ru Subject: Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: 'Kevin Wolf' Cc: edgar.iglesias@xilinx.com, peter.maydell@linaro.org, igor.rubinov@gmail.com, mark.burton@greensocs.com, real@ispras.ru, hines@cert.org, qemu-devel@nongnu.org, maria.klimushenkova@ispras.ru, stefanha@redhat.com, pbonzini@redhat.com, batuzovk@ispras.ru, alex.bennee@linaro.org, fred.konrad@greensocs.com > From: Pavel Dovgalyuk [mailto: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. 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