From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161131AbXCHXnZ (ORCPT ); Thu, 8 Mar 2007 18:43:25 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1161221AbXCHXnY (ORCPT ); Thu, 8 Mar 2007 18:43:24 -0500 Received: from gw1.cosmosbay.com ([86.65.150.130]:56533 "EHLO gw1.cosmosbay.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161131AbXCHXnY (ORCPT ); Thu, 8 Mar 2007 18:43:24 -0500 Message-ID: <45F09F9C.4030801@cosmosbay.com> Date: Fri, 09 Mar 2007 00:43:24 +0100 From: Eric Dumazet User-Agent: Thunderbird 1.5.0.10 (Windows/20070221) MIME-Version: 1.0 To: "Michael K. Edwards" CC: Linux Kernel Mailing List Subject: Re: sys_write() racy for multi-threaded append? References: In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-1.6 (gw1.cosmosbay.com [86.65.150.130]); Fri, 09 Mar 2007 00:43:21 +0100 (CET) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Michael K. Edwards a écrit : > from sys_write(): > > file = fget_light(fd, &fput_needed); > if (file) { > loff_t pos = file_pos_read(file); > ret = vfs_write(file, buf, count, &pos); > file_pos_write(file, pos); > fput_light(file, fput_needed); > } > > Surely that's racy when two threads write to the same fd and the first > vfs_write call blocks or is preempted. Or does fget_light take some > per-file lock that I'm not seeing? Nothing in the manuals says that write() on same fd should be non racy : In particular file pos might be undefined. There is a reason pwrite() exists. Kernel doesnt have to enforce thread safety as standard is quite clear. Only O_APPEND case is specially handled (and NFS might fail to handle this case correctly)