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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 3AD7DC433E0 for ; Fri, 26 Feb 2021 01:23:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E551664EE4 for ; Fri, 26 Feb 2021 01:23:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229934AbhBZBXk (ORCPT ); Thu, 25 Feb 2021 20:23:40 -0500 Received: from mail.kernel.org ([198.145.29.99]:53010 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230095AbhBZBWz (ORCPT ); Thu, 25 Feb 2021 20:22:55 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 6E5D864EE4; Fri, 26 Feb 2021 01:22:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1614302547; bh=jKB901BQIYoU4zp3CyOHUYXxA7nUUV5QF+6E8s0LEXI=; h=Date:From:To:Subject:In-Reply-To:From; b=CVW8kwEXCaX/Te74zcCKz52B9zSiXKwxrkrf2jb4462CsYracict7PBK4jy+mp+47 aQYEePpiJmP0CUunumddXqQRrGE6aMOw3NhcFFaM5+Zk8if1EdkVW+WWN1ZQZta455 Uf399bRYY2VuQxEVj4717sDMg+xtbxMdVj91/iZ4= Date: Thu, 25 Feb 2021 17:22:25 -0800 From: Andrew Morton To: akpm@linux-foundation.org, corbet@lwn.net, dave.hansen@linux.intel.com, davem@davemloft.net, linux-mm@kvack.org, lucien.xin@gmail.com, luto@kernel.org, marcelo.leitner@gmail.com, mingo@redhat.com, mm-commits@vger.kernel.org, neilb@suse.de, nhorman@tuxdriver.com, peterz@infradead.org, stable@vger.kernel.org, torvalds@linux-foundation.org, viro@zeniv.linux.org.uk, vyasevich@gmail.com Subject: [patch 112/118] seq_file: document how per-entry resources are managed. Message-ID: <20210226012225.-bfePYsH9%akpm@linux-foundation.org> In-Reply-To: <20210225171452.713967e96554bb6a53e44a19@linux-foundation.org> User-Agent: s-nail v14.8.16 Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org From: NeilBrown Subject: seq_file: document how per-entry resources are managed. Patch series "Fix some seq_file users that were recently broken". A recent change to seq_file broke some users which were using seq_file in a non-"standard" way ... though the "standard" isn't documented, so they can be excused. The result is a possible leak - of memory in one case, of references to a 'transport' in the other. These three patches: 1/ document and explain the problem 2/ fix the problem user in x86 3/ fix the problem user in net/sctp This patch (of 3): Users of seq_file will sometimes find it convenient to take a resource, such as a lock or memory allocation, in the ->start or ->next operations. These are per-entry resources, distinct from per-session resources which are taken in ->start and released in ->stop. The preferred management of these is release the resource on the subsequent call to ->next or ->stop. However prior to Commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface") it happened that ->show would always be called after ->start or ->next, and a few users chose to release the resource in ->show. This is no longer reliable. Since the mentioned commit, ->next will always come after a successful ->show (to ensure m->index is updated correctly), so the original ordering cannot be maintained. This patch updates the documentation to clearly state the required behaviour. Other patches will fix the few problematic users. [akpm@linux-foundation.org: fix typo, per Willy] Link: https://lkml.kernel.org/r/161248518659.21478.2484341937387294998.stgit@noble1 Link: https://lkml.kernel.org/r/161248539020.21478.3147971477400875336.stgit@noble1 Fixes: 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface") Signed-off-by: NeilBrown Cc: Xin Long Cc: Alexander Viro Cc: Jonathan Corbet Cc: Ingo Molnar Cc: Dave Hansen Cc: Andy Lutomirski Cc: Peter Zijlstra Cc: Vlad Yasevich Cc: Neil Horman Cc: Marcelo Ricardo Leitner Cc: "David S. Miller" Cc: Signed-off-by: Andrew Morton --- Documentation/filesystems/seq_file.rst | 6 ++++++ 1 file changed, 6 insertions(+) --- a/Documentation/filesystems/seq_file.rst~seq_file-document-how-per-entry-resources-are-managed +++ a/Documentation/filesystems/seq_file.rst @@ -217,6 +217,12 @@ between the calls to start() and stop(), is a reasonable thing to do. The seq_file code will also avoid taking any other locks while the iterator is active. +The iterater value returned by start() or next() is guaranteed to be +passed to a subsequent next() or stop() call. This allows resources +such as locks that were taken to be reliably released. There is *no* +guarantee that the iterator will be passed to show(), though in practice +it often will be. + Formatted output ================ _