From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_2 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7F78EC38A24 for ; Thu, 7 May 2020 16:36:27 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 5110820643 for ; Thu, 7 May 2020 16:36:27 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5110820643 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kaod.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:43296 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jWjVa-0002sS-C1 for qemu-devel@archiver.kernel.org; Thu, 07 May 2020 12:36:26 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:44406) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jWjUy-0002TY-9x for qemu-devel@nongnu.org; Thu, 07 May 2020 12:35:48 -0400 Received: from 2.mo6.mail-out.ovh.net ([46.105.76.65]:53142) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jWjUw-0003l9-4r for qemu-devel@nongnu.org; Thu, 07 May 2020 12:35:48 -0400 Received: from player761.ha.ovh.net (unknown [10.108.42.167]) by mo6.mail-out.ovh.net (Postfix) with ESMTP id 4847920EAB0 for ; Thu, 7 May 2020 18:35:42 +0200 (CEST) Received: from kaod.org (lns-bzn-46-82-253-208-248.adsl.proxad.net [82.253.208.248]) (Authenticated sender: groug@kaod.org) by player761.ha.ovh.net (Postfix) with ESMTPSA id 9CA061224844D; Thu, 7 May 2020 16:35:40 +0000 (UTC) Date: Thu, 7 May 2020 18:35:36 +0200 From: Greg Kurz To: Christian Schoenebeck Subject: Re: [PATCH] 9pfs: Fix potential deadlock of QEMU mainloop Message-ID: <20200507183536.1714287f@bahia.lan> In-Reply-To: <3839530.O0e2CIhMhP@silver> References: <158826201391.1344781.9403916162733181811.stgit@bahia.lan> <8590081.eFxiLWWr9E@silver> <20200507163328.4736534d@bahia.lan> <3839530.O0e2CIhMhP@silver> X-Mailer: Claws Mail 3.17.4 (GTK+ 2.24.32; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Ovh-Tracer-Id: 4097431238336026944 X-VR-SPAMSTATE: OK X-VR-SPAMSCORE: 0 X-VR-SPAMCAUSE: gggruggvucftvghtrhhoucdtuddrgeduhedrkedtgddutddtucetufdoteggodetrfdotffvucfrrhhofhhilhgvmecuqfggjfdpvefjgfevmfevgfenuceurghilhhouhhtmecuhedttdenucenucfjughrpeffhffvuffkjghfofggtgfgsehtjeertdertddvnecuhfhrohhmpefirhgvghcumfhurhiiuceoghhrohhugheskhgrohgurdhorhhgqeenucggtffrrghtthgvrhhnpeehkefhtdehgeehheejledufeekhfdvleefvdeihefhkefhudffhfeuuedvffdthfenucfkpheptddrtddrtddrtddpkedvrddvheefrddvtdekrddvgeeknecuvehluhhsthgvrhfuihiivgeptdenucfrrghrrghmpehmohguvgepshhmthhpqdhouhhtpdhhvghlohepphhlrgihvghrjeeiuddrhhgrrdhovhhhrdhnvghtpdhinhgvtheptddrtddrtddrtddpmhgrihhlfhhrohhmpehgrhhouhhgsehkrghougdrohhrghdprhgtphhtthhopehqvghmuhdquggvvhgvlhesnhhonhhgnhhurdhorhhg Received-SPF: pass client-ip=46.105.76.65; envelope-from=groug@kaod.org; helo=2.mo6.mail-out.ovh.net X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/07 12:35:42 X-ACL-Warn: Detected OS = Linux 3.11 and newer X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, 07 May 2020 17:03:46 +0200 Christian Schoenebeck wrote: > On Donnerstag, 7. Mai 2020 16:33:28 CEST Greg Kurz wrote: > > > I also haven't reviewed QEMU's lock implementations in very detail, but > > > IIRC CoMutexes are completely handled in user space, while QemuMutex uses > > > regular OS mutexes and hence might cost context switches. > > > > ... since the locking would only been exercised with an hypothetical > > client doing stupid things, this is beginning to look like bike-shedding > > to me. :) > > Aha, keep that in mind when you're doing your next review. ;-) > Fair enough. :) > No seriously, like I said, I don't really care too much about Mutex vs. > CoMutex in you patch here. It was actually more about wide-picture thinking, > i.e. other places of (co)mutexes being used or other potential changes that > would make this or other uses more relevant one day. > Then I agree with you that it would be better to use CoMutex if we were experiencing thread pool exhaustion indeed. > > > > > > diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c > > > > > > index 9e046f7acb51..ac84ae804496 100644 > > > > > > --- a/hw/9pfs/9p.c > > > > > > +++ b/hw/9pfs/9p.c > > > > > > @@ -2170,7 +2170,7 @@ static int coroutine_fn > > > > > > v9fs_do_readdir_with_stat(V9fsPDU *pdu, int32_t count = 0; > > > > > > > > > > > > struct stat stbuf; > > > > > > off_t saved_dir_pos; > > > > > > > > > > > > - struct dirent *dent; > > > > > > + struct dirent dent; > > > > > > > > > > > > /* save the directory position */ > > > > > > saved_dir_pos = v9fs_co_telldir(pdu, fidp); > > > > > > > > > > > > @@ -2181,13 +2181,11 @@ static int coroutine_fn > > > > > > v9fs_do_readdir_with_stat(V9fsPDU *pdu, while (1) { > > > > > > > > > > > > v9fs_path_init(&path); > > > > > > > > > > > > - v9fs_readdir_lock(&fidp->fs.dir); > > > > > > - > > > > > > > > > > That's the deadlock fix, but ... > > > > > > > > > > > err = v9fs_co_readdir(pdu, fidp, &dent); > > > > > > > > > > > > - if (err || !dent) { > > > > > > + if (err <= 0) { > > > > > > > > > > > > break; > > > > > > > > > > > > } > > > > > > > > > > ... even though this code simplification might make sense, I don't > > > > > think > > > > > it > > > > > should be mixed with the deadlock fix together in one patch. They are > > > > > not > > > > > > > > I could possibly split this in two patches, one for returning a copy > > > > and one for moving the locking around, but... > > > > > > > > > related with each other, nor is the code simplification you are aiming > > > > > trivial > > > > > > > > ... this assertion is somewhat wrong: moving the locking to > > > > v9fs_co_readdir() really requires it returns a copy. > > > > > > Yeah, I am also not sure whether a split would make it more trivial enough > > > in this case to be worth the hassle. If you find an acceptable solution, > > > good, if not then leave it one patch. > > > > Another option would be to g_malloc() the dirent in v9fs_co_readdir() and > > g_free() in the callers. This would cause less churn since we could keep > > the same function signature. > > I was actually just going to suggest the same. So yes, looks like a less > invasive change to me. > I'll just do that then :) > Best regards, > Christian Schoenebeck > >