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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,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 63890C04AB6 for ; Fri, 31 May 2019 11:23:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 46BAA2682D for ; Fri, 31 May 2019 11:23:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726331AbfEaLW4 (ORCPT ); Fri, 31 May 2019 07:22:56 -0400 Received: from mx2.suse.de ([195.135.220.15]:51444 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726002AbfEaLW4 (ORCPT ); Fri, 31 May 2019 07:22:56 -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 09B2EAD05; Fri, 31 May 2019 11:22:55 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Fri, 31 May 2019 13:22:54 +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 07/13] epoll: call ep_add_event_to_uring() from ep_poll_callback() In-Reply-To: <20190531095616.GD17637@hirez.programming.kicks-ass.net> References: <20190516085810.31077-1-rpenyaev@suse.de> <20190516085810.31077-8-rpenyaev@suse.de> <20190531095616.GD17637@hirez.programming.kicks-ass.net> Message-ID: <98971429dc36e8a2e3417af1744de2b2@suse.de> 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 11:56, Peter Zijlstra wrote: > On Thu, May 16, 2019 at 10:58:04AM +0200, Roman Penyaev wrote: >> Each ep_poll_callback() is called when fd calls wakeup() on epfd. >> So account new event in user ring. >> >> The tricky part here is EPOLLONESHOT. Since we are lockless we >> have to be deal with ep_poll_callbacks() called in paralle, thus >> use cmpxchg to clear public event bits and filter out concurrent >> call from another cpu. >> >> Signed-off-by: Roman Penyaev >> Cc: Andrew Morton >> Cc: Al Viro >> Cc: Linus Torvalds >> Cc: linux-fsdevel@vger.kernel.org >> Cc: linux-kernel@vger.kernel.org >> >> diff --git a/fs/eventpoll.c b/fs/eventpoll.c >> index 2f551c005640..55612da9651e 100644 >> --- a/fs/eventpoll.c >> +++ b/fs/eventpoll.c >> @@ -1407,6 +1407,29 @@ struct file *get_epoll_tfile_raw_ptr(struct >> file *file, int tfd, >> } >> #endif /* CONFIG_CHECKPOINT_RESTORE */ >> >> +/** >> + * Atomically clear public event bits and return %true if the old >> value has >> + * public event bits set. >> + */ >> +static inline bool ep_clear_public_event_bits(struct epitem *epi) >> +{ >> + __poll_t old, flags; >> + >> + /* >> + * Here we race with ourselves and with ep_modify(), which can >> + * change the event bits. In order not to override events updated >> + * by ep_modify() we have to do cmpxchg. >> + */ >> + >> + old = epi->event.events; >> + do { >> + flags = old; >> + } while ((old = cmpxchg(&epi->event.events, flags, >> + flags & EP_PRIVATE_BITS)) != flags); >> + >> + return flags & ~EP_PRIVATE_BITS; >> +} > > AFAICT epi->event.events also has normal writes to it, eg. in > ep_modify(). A number of architectures cannot handle concurrent normal > writes and cmpxchg() to the same variable. Yes, we race with the current function and with ep_modify(). Then, ep_modify() should do something as the following: - epi->event.events = event->events + xchg(&epi->event.events, event->events); Is that ok? Just curious: what are these archs? Thanks. -- Roman