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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 01BD3C3A59E for ; Mon, 26 Aug 2019 06:22:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CB5712173E for ; Mon, 26 Aug 2019 06:22:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729629AbfHZGWa (ORCPT ); Mon, 26 Aug 2019 02:22:30 -0400 Received: from dcvr.yhbt.net ([64.71.152.64]:52088 "EHLO dcvr.yhbt.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729535AbfHZGWa (ORCPT ); Mon, 26 Aug 2019 02:22:30 -0400 Received: from localhost (dcvr.yhbt.net [127.0.0.1]) by dcvr.yhbt.net (Postfix) with ESMTP id D53A21F461; Mon, 26 Aug 2019 06:22:29 +0000 (UTC) Date: Mon, 26 Aug 2019 06:22:29 +0000 From: Eric Wong To: Heiher Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: Why the edge-triggered mode doesn't work for epoll file descriptor? Message-ID: <20190826062229.pjvumg4ag3qwhji6@whir> References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Heiher wrote: > Hello, > > I've added a pipe file descriptor (fd1) to an epoll (fd3) with > EPOLLOUT in edge-triggered mode, and then added the fd3 to another > epoll (fd4) with EPOLLIN in edge-triggered too. > > Next, waiting for fd4 without timeout. When fd1 to be writable, i > think epoll_wait(fd4, ...) only return once, because all file > descriptors are added in edge-triggered mode. > > But, the actual result is returns many and many times until do once > eopll_wait(fd3, ...). It looks like you can trigger a wakeup loop with printf writing to the terminal (not a pipe), and that write to the terminal triggering the EPOLLOUT wakeup over and over again. I don't know TTY stuff at all, but I assume it's intended for terminals. You refer to "pipe file descriptor (fd1)", but I can't reproduce the error when running your code piped to "tee" and using strace to check epoll_wait returns. "strace ./foo | tee /dev/null" only shows one epoll_wait returning. > e.events = EPOLLIN | EPOLLET; > e.data.u64 = 1; > if (epoll_ctl (efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0) > return -3; > > e.events = EPOLLOUT | EPOLLET; > e.data.u64 = 2; > if (epoll_ctl (efd[1], EPOLL_CTL_ADD, 1, &e) < 0) > return -4; Since epfd[1] is waiting for stdout... > for (;;) { > struct epoll_event events[16]; > int nfds; > > nfds = epoll_wait (efd[0], events, 16, -1); > printf ("nfds: %d\n", nfds); Try outputting your message to stderr instead of stdout: fprintf(stderr, "nfds: %d\n", nfds); And then run your program so stdout and stderr point to different files: ./foo | tee /dev/null (so stdout becomes a pipe, and stderr remains your terminal)