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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable 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 479C7C4360F for ; Thu, 4 Apr 2019 18:44:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 23CC220657 for ; Thu, 4 Apr 2019 18:44:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729836AbfDDSoy (ORCPT ); Thu, 4 Apr 2019 14:44:54 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:52316 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729456AbfDDSoy (ORCPT ); Thu, 4 Apr 2019 14:44:54 -0400 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92 #3 (Red Hat Linux)) id 1hC7M0-0004An-VA; Thu, 04 Apr 2019 18:44:49 +0000 Date: Thu, 4 Apr 2019 19:44:48 +0100 From: Al Viro To: Amir Goldstein Cc: Miklos Szeredi , Dmitry Vyukov , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com Subject: Re: [PATCH v2] acct: fix possible deadlock in acct_pin_kill Message-ID: <20190404184448.GC2217@ZenIV.linux.org.uk> References: <20190404105255.12189-1-amir73il@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190404105255.12189-1-amir73il@gmail.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Apr 04, 2019 at 01:52:55PM +0300, Amir Goldstein wrote: > This looks like an old bug, pre-dating the "Fixes" commit, but the > "Fixes" commit did not handle it properly. > > The bug recently surfaced as a lockdep possible deadlock warning > with commit d1d04ef8572b ("ovl: stack file ops"). > > When acct_on() replaces one acct file with another, it takes sb_writers > lock on new file sb and calls acct_pin_kill(old) before releasing the > sb_writers lock. > > If new file is on the same fs as old file, acct_pin_kill(old) fail to > file_start_write_trylock() and skip writing the old file, because > sb_writers (of new) is already taken by acct_on(). > > If new file is not on same fs as old file, this ordering violation > creates an unneeded dependency between new sb_writers and old sb_writers, > which may later be reported as possible deadlock. > > This could result in an actual deadlock if acct file is replaced from > an old file in overlayfs over "real fs" to a new file in "real fs". > acct_on() takes freeze protection on "real fs" and tries to write to > overlayfs file. overlayfs is not freeze protected so do_acct_process() > can carry on with __kernel_write() to overlayfs file, which would > try to take freeze protection on "real fs" and deadlock. Huh? sb_writers is taken when we *open* the new file. Then we replace its ->path.mnt with a clone and transfer the write count from the original to new one. And close the old file while we are at it. >From sb_writers POV mainline has sb_start_write(new_sb) // in open sb_start_write(new_sb) // mnt_want_write() on clone last write to old_sb, then sb_end_write(old_sb) // acct_pin_kill() sb_end_write(new_sb) // mnt_drop_write(mnt) and you flip the order of the last two lines. Could you explain how exactly does your patch help whatever problem overlayfs has?