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_HELO_NONE,SPF_PASS 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 CACEFC04AB6 for ; Fri, 31 May 2019 14:28:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ACA4226A9A for ; Fri, 31 May 2019 14:28:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726609AbfEaO2i (ORCPT ); Fri, 31 May 2019 10:28:38 -0400 Received: from mx2.suse.de ([195.135.220.15]:59752 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726418AbfEaO2i (ORCPT ); Fri, 31 May 2019 10:28:38 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id EE5E1AF99; Fri, 31 May 2019 14:28:36 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Fri, 31 May 2019 16:28:36 +0200 From: Roman Penyaev To: Peter Zijlstra Cc: azat@libevent.org, akpm@linux-foundation.org, viro@zeniv.linux.org.uk, torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 06/13] epoll: introduce helpers for adding/removing events to uring In-Reply-To: <20190531125322.GY2606@hirez.programming.kicks-ass.net> References: <20190516085810.31077-1-rpenyaev@suse.de> <20190516085810.31077-7-rpenyaev@suse.de> <20190531095607.GC17637@hirez.programming.kicks-ass.net> <274e29d102133f3be1f309c66cb0af36@suse.de> <20190531125322.GY2606@hirez.programming.kicks-ass.net> Message-ID: X-Sender: rpenyaev@suse.de User-Agent: Roundcube Webmail Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 2019-05-31 14:53, Peter Zijlstra wrote: > On Fri, May 31, 2019 at 01:15:21PM +0200, Roman Penyaev wrote: >> On 2019-05-31 11:56, Peter Zijlstra wrote: >> > On Thu, May 16, 2019 at 10:58:03AM +0200, Roman Penyaev wrote: > >> > > + i = __atomic_fetch_add(&ep->user_header->tail, 1, >> > > + __ATOMIC_ACQUIRE); >> > >> > afaict __atomic_fetch_add() does not exist. >> >> That is gcc extension. I did not find any API just to increment >> the variable atomically without using/casting to atomic. What >> is a proper way to achieve that? > > That's C11 atomics, and those shall not be used in the kernel. For one > they're not available in the minimally required GCC version (4.6). > > The proper and only way is to use atomic_t, but also you cannot share > atomic_t with userspace. Yes, that what I tried to avoid choosing c11 extension. > > The normal way of doing something like this is to have a kernel private > atomic_t and copy the value out to userspace using smp_store_release(). Since this path is lockless unfortunately that won't work. So seems the only way is to do one more cmpxchg (sigh) or give up and take a look (sad sigh). -- Roman