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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 D476FC04EB9 for ; Wed, 5 Dec 2018 11:25:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 742C320659 for ; Wed, 5 Dec 2018 11:25:45 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 742C320659 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.de Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727389AbeLELZo (ORCPT ); Wed, 5 Dec 2018 06:25:44 -0500 Received: from mx2.suse.de ([195.135.220.15]:59564 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726979AbeLELZn (ORCPT ); Wed, 5 Dec 2018 06:25:43 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 036C1B002; Wed, 5 Dec 2018 11:25:41 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Wed, 05 Dec 2018 12:25:39 +0100 From: Roman Penyaev To: Andrea Parri Cc: Linus Torvalds , Al Viro , Paul McKenney , linux-fsdevel , Linux List Kernel Mailing Subject: Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce ep_poll_callback() contention In-Reply-To: <20181204235907.GA5388@andrea> References: <20181203110237.14787-1-rpenyaev@suse.de> <83edf06ce9db540495b53527eca3248c@suse.de> <20181204235907.GA5388@andrea> Message-ID: <51e8202b163d7522efb18aa2ca79d952@suse.de> X-Sender: rpenyaev@suse.de User-Agent: Roundcube Webmail Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2018-12-05 00:59, Andrea Parri wrote: > Hi Roman, > > On Tue, Dec 04, 2018 at 12:50:58PM +0100, Roman Penyaev wrote: >> On 2018-12-03 18:34, Linus Torvalds wrote: > >> > This also ends up making the memory ordering of "xchg()" very very >> > important. Yes, we've documented it as being an ordering op, but I'm >> > not sure we've relied on it this directly before. >> >> Seems exit_mm() does exactly the same, the following chunk: >> >> up_read(&mm->mmap_sem); >> >> self.task = current; >> self.next = xchg(&core_state->dumper.next, &self); >> >> >> At least code pattern looks similar. > > Maybe add a comment on top of (your) xchg() to note/justify these > memory > ordering requirements? As Paul said: "if there are races, this would > help force them to happen" (and simplify the review, this/future). Hi Andrea, Sure, this path is tricky, so will I cook something up. -- Roman