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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 4B6BFC3A5A2 for ; Tue, 3 Sep 2019 15:41:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2ED5D2077B for ; Tue, 3 Sep 2019 15:41:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729644AbfICPlR (ORCPT ); Tue, 3 Sep 2019 11:41:17 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:59404 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729109AbfICPlQ (ORCPT ); Tue, 3 Sep 2019 11:41:16 -0400 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.1 #3 (Red Hat Linux)) id 1i5Avi-0007j7-2D; Tue, 03 Sep 2019 15:41:14 +0000 Date: Tue, 3 Sep 2019 16:41:14 +0100 From: Al Viro To: "zhengbin (A)" Cc: jack@suse.cz, akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, "zhangyi (F)" Subject: Re: Possible FS race condition between iterate_dir and d_alloc_parallel Message-ID: <20190903154114.GK1131@ZenIV.linux.org.uk> References: <20190903154007.GJ1131@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190903154007.GJ1131@ZenIV.linux.org.uk> User-Agent: Mutt/1.12.0 (2019-05-25) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Sep 03, 2019 at 04:40:07PM +0100, Al Viro wrote: > On Tue, Sep 03, 2019 at 10:44:32PM +0800, zhengbin (A) wrote: > > We recently encountered an oops(the filesystem is tmpfs) > > crash> bt > > #9 [ffff0000ae77bd60] dcache_readdir at ffff0000672954bc > > > > The reason is as follows: > > Process 1 cat test which is not exist in directory A, process 2 cat test in directory A too. > > process 3 create new file in directory B, process 4 ls directory A. > > > good grief, what screen width do you have to make the table below readable? > > What I do not understand is how the hell does your dtry2 manage to get actually > freed and reused without an RCU delay between its removal from parent's > ->d_subdirs and freeing its memory. What should've happened in that > scenario is > * process 4, in next_positive() grabs rcu_read_lock(). > * it walks into your dtry2, which might very well be > just a chunk of memory waiting to be freed; it sure as hell is > not positive. skipped is set to true, 'i' is not decremented. > Note that ->d_child.next points to the next non-cursor sibling > (if any) or to the ->d_subdir of parent, so we can keep walking. > * we keep walking for a while; eventually we run out of > counter and leave the loop. > > Only after that we do rcu_read_unlock() and only then anything > observed in that loop might be freed and reused. > > Confused... OTOH, I might be misreading that table of yours - > it's about 30% wider than the widest xterm I can get while still > being able to read the font... Incidentally, which kernel was that on?