From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf0-f71.google.com (mail-lf0-f71.google.com [209.85.215.71]) by kanga.kvack.org (Postfix) with ESMTP id 512226B0006 for ; Mon, 26 Mar 2018 17:29:48 -0400 (EDT) Received: by mail-lf0-f71.google.com with SMTP id 91-v6so6468626lfu.20 for ; Mon, 26 Mar 2018 14:29:48 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id d8sor2994442ljj.89.2018.03.26.14.29.46 for (Google Transport Security); Mon, 26 Mar 2018 14:29:46 -0700 (PDT) Date: Tue, 27 Mar 2018 00:29:44 +0300 From: Cyrill Gorcunov Subject: Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct Message-ID: <20180326212944.GF2236@uranus> References: <1522088439-105930-1-git-send-email-yang.shi@linux.alibaba.com> <20180326183725.GB27373@bombadil.infradead.org> <20180326192132.GE2236@uranus> <0bfa8943-a2fe-b0ab-99a2-347094a2bcec@i-love.sakura.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0bfa8943-a2fe-b0ab-99a2-347094a2bcec@i-love.sakura.ne.jp> Sender: owner-linux-mm@kvack.org List-ID: To: Tetsuo Handa Cc: Matthew Wilcox , Yang Shi , adobriyan@gmail.com, mhocko@kernel.org, mguzik@redhat.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On Tue, Mar 27, 2018 at 06:10:09AM +0900, Tetsuo Handa wrote: > On 2018/03/27 4:21, Cyrill Gorcunov wrote: > > That said I think using read-lock here would be a bug. > > If I understand correctly, the caller can't set both fields atomically, for > prctl() does not receive both fields at one call. > > prctl(PR_SET_MM, PR_SET_MM_ARG_START xor PR_SET_MM_ARG_END xor PR_SET_MM_ENV_START xor PR_SET_MM_ENV_END, new value, 0, 0); > True, but the key moment is that two/three/four system calls can run simultaneously. And while previously they are ordered by "write", with read lock they are completely unordered and this is really worries me. To be fair I would prefer to drop this old per-field interface completely. This per-field interface was rather an ugly solution from my side. > Then, I wonder whether reading arg_start|end and env_start|end atomically makes > sense. Just retry reading if arg_start > env_end or env_start > env_end is fine? Tetsuo, let me re-read this code tomorrow, maybe I miss something obvious.