From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f72.google.com (mail-pl0-f72.google.com [209.85.160.72]) by kanga.kvack.org (Postfix) with ESMTP id 0C9136B0025 for ; Tue, 27 Mar 2018 14:44:23 -0400 (EDT) Received: by mail-pl0-f72.google.com with SMTP id b2-v6so15953643plz.17 for ; Tue, 27 Mar 2018 11:44:23 -0700 (PDT) Received: from out30-130.freemail.mail.aliyun.com (out30-130.freemail.mail.aliyun.com. [115.124.30.130]) by mx.google.com with ESMTPS id q9-v6si1799735plr.21.2018.03.27.11.44.21 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 27 Mar 2018 11:44:21 -0700 (PDT) Subject: Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct References: <1522088439-105930-1-git-send-email-yang.shi@linux.alibaba.com> <20180326183725.GB27373@bombadil.infradead.org> <20180326192132.GE2236@uranus> <20180327073212.GG2236@uranus> From: Yang Shi Message-ID: <73559b36-b55b-429a-285f-c05b45129b51@linux.alibaba.com> Date: Tue, 27 Mar 2018 14:44:09 -0400 MIME-Version: 1.0 In-Reply-To: <20180327073212.GG2236@uranus> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: To: Cyrill Gorcunov Cc: Matthew Wilcox , adobriyan@gmail.com, mhocko@kernel.org, mguzik@redhat.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 3/27/18 3:32 AM, Cyrill Gorcunov wrote: > On Mon, Mar 26, 2018 at 05:59:49PM -0400, Yang Shi wrote: >>> Say we've two syscalls running prctl_set_mm_map in parallel, and imagine >>> one have @start_brk = 20 @brk = 10 and second caller has @start_brk = 30 >>> and @brk = 20. Since now the call is guarded by _read_ the both calls >>> unlocked and due to OO engine it may happen then when both finish >>> we have @start_brk = 30 and @brk = 10. In turn "write" semaphore >>> has been take to have consistent data on exit, either you have [20;10] >>> or [30;20] assigned not something mixed. >>> >>> That said I think using read-lock here would be a bug. >> Yes it sounds so. However, it was down_read before >> ddf1d398e517e660207e2c807f76a90df543a217 ("prctl: take mmap sem for writing >> to protect against others"). And, that commit is for fixing the concurrent >> writing to arg_* and env_*. I just checked that commit, but omitted the brk >> part. The potential issue mentioned by you should exist before that commit, >> but might be just not discovered or very rare to hit. >> >> I will change it back to down_write. > down_read before was a bug ;) And it was not discovered earlier simply > because not that many users of this interface exist, namely only criu > as far as I know by now. Thanks for confirming this. I assumed so :-)