From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38371) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b0sXd-0005Iz-KJ for qemu-devel@nongnu.org; Thu, 12 May 2016 11:28:47 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1b0sXa-000129-QK for qemu-devel@nongnu.org; Thu, 12 May 2016 11:28:44 -0400 Date: Thu, 12 May 2016 17:28:34 +0200 From: Kevin Wolf Message-ID: <20160512152834.GG4794@noname.redhat.com> References: <5720BFDB.60600@redhat.com> <20160429151826.GM4350@noname.redhat.com> <20160503132324.GE3917@noname.str.redhat.com> <20160503134847.GH3917@noname.str.redhat.com> <20160512150451.GF4794@noname.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alberto Garcia Cc: Max Reitz , qemu-devel@nongnu.org, qemu-block@nongnu.org, Eric Blake , Stefan Hajnoczi Am 12.05.2016 um 17:13 hat Alberto Garcia geschrieben: > On Thu 12 May 2016 05:04:51 PM CEST, Kevin Wolf wrote: > > Am 12.05.2016 um 15:47 hat Alberto Garcia geschrieben: > >> On Tue 03 May 2016 03:48:47 PM CEST, Kevin Wolf wrote: > >> > Am 03.05.2016 um 15:33 hat Alberto Garcia geschrieben: > >> >> On Tue 03 May 2016 03:23:24 PM CEST, Kevin Wolf wrote: > >> >> >> c) we fix bdrv_reopen() so we can actually run both jobs at the same > >> >> >> time. I'm wondering if pausing all block jobs between > >> >> >> bdrv_reopen_prepare() and bdrv_reopen_commit() would do the > >> >> >> trick. Opinions? > >> >> > > >> >> > I would have to read up the details of the problem again, but I think > >> >> > with bdrv_drained_begin/end() we actually have the right tool now to fix > >> >> > it properly. We may need to pull up the drain (bdrv_drain_all() today) > >> >> > from bdrv_reopen_multiple() to its caller and just assert it in the > >> >> > function itself, but there shouldn't be much more to it than that. > >> >> > >> >> I think that's not enough, see point 2) here: > >> >> > >> >> https://lists.gnu.org/archive/html/qemu-block/2015-12/msg00180.html > >> >> > >> >> "I've been taking a look at the bdrv_drained_begin/end() API, but as I > >> >> understand it it prevents requests from a different AioContext. > >> >> Since all BDS in the same chain share the same context it does not > >> >> really help here." > >> > > >> > Yes, that's the part I meant with pulling up the calls. > >> > > >> > If I understand correctly, the problem is that first bdrv_reopen_queue() > >> > queues a few BDSes for reopen, then bdrv_drain_all() completes all > >> > running requests and can indirectly trigger a graph modification, and > >> > then bdrv_reopen_multiple() uses the queue which doesn't match reality > >> > any more. > >> > > >> > The solution to that should be simply changing the order of things: > >> > > >> > 1. bdrv_drained_begin() > >> > 2. bdrv_reopen_queue() > >> > 3. bdrv_reopen_multiple() > >> > * Instead of bdrv_drain_all(), assert that no requests are pending > >> > * We don't run requests, so we can't complete a block job and > >> > manipulate the graph any more > >> > 4. then bdrv_drained_end() > >> > >> This doesn't work. Here's what happens: > >> > >> 1) Block job (a) starts (block-stream). > >> > >> 2) Block job (b) starts (block-stream, or block-commit). > >> > >> 3) job (b) calls bdrv_reopen() and does the drain call. > >> > >> 4) job (b) creates reopen_queue and calls bdrv_reopen_multiple(). > >> There are no pending requests at this point, but job (a) is sleeping. > >> > >> 5) bdrv_reopen_multiple() iterates over reopen_queue and calls > >> bdrv_reopen_prepare() -> bdrv_flush() -> bdrv_co_flush() -> > >> qemu_coroutine_yield(). > > > > I think between here and the next step is what I don't understand. > > > > bdrv_reopen_multiple() is not called in coroutine context, right? All > > block jobs use block_job_defer_to_main_loop() before they call > > bdrv_reopen(), as far as I can see. So bdrv_flush() shouldn't take the > > shortcut, but use a nested event loop. > > When bdrv_flush() is not called in coroutine context it does > qemu_coroutine_create() + qemu_coroutine_enter(). Right, but if the coroutine yields, we jump back to the caller, which looks like this: co = qemu_coroutine_create(bdrv_flush_co_entry); qemu_coroutine_enter(co, &rwco); while (rwco.ret == NOT_DONE) { aio_poll(aio_context, true); } So this loops until the flush has completed. The only way I can see how something else (job (a)) can run is if aio_poll() calls it. > > What is it that calls into job (a) from that event loop? It can't be a > > request completion because we already drained all requests. Is it a > > timer? > > If I didn't get it wrong it's this bit in bdrv_co_flush() > [...] That's the place that yields from (b), but it's not the place that calls into (a). > >> 6) job (a) resumes, finishes the job and removes nodes from the graph. > >> > >> 7) job (b) continues with bdrv_reopen_multiple() but now reopen_queue > >> contains invalid pointers. > > > > I don't fully understand the problem yet, but as a shot in the dark, > > would pausing block jobs in bdrv_drained_begin() help? > > Yeah, my impression is that pausing all jobs during bdrv_reopen() should > be enough. If you base your patches on top of my queue (which I think you already do for the greatest part), the nicest way to implement this would probably be that BlockBackends give their users a callback for .drained_begin/end and the jobs implement that as pausing themselves. We could, of course, directly pause block jobs in bdrv_drained_begin(), but that would feel a bit hackish. So maybe do that for a quick attempt whether it helps, and if it does, we can write the real thing then. Kevin