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=-6.6 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,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 DADBCC00A89 for ; Thu, 5 Nov 2020 12:21:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 72EB120729 for ; Thu, 5 Nov 2020 12:21:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="OazK+A8k" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730479AbgKEMVl (ORCPT ); Thu, 5 Nov 2020 07:21:41 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52662 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725468AbgKEMVk (ORCPT ); Thu, 5 Nov 2020 07:21:40 -0500 Received: from mail-lf1-x143.google.com (mail-lf1-x143.google.com [IPv6:2a00:1450:4864:20::143]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C8277C0613CF; Thu, 5 Nov 2020 04:21:39 -0800 (PST) Received: by mail-lf1-x143.google.com with SMTP id h6so2008141lfj.3; Thu, 05 Nov 2020 04:21:39 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=nl8/XcSo5npDTXvRYjKf28LcA8OJMzH5M7EJrj7FCrE=; b=OazK+A8kxglaaIuq2KGJ4seXC7YezLIYXArpgFKHCPuJGB5FY6rBtFmGwRirWINIo/ jGvtqgbVj+Y2vpn6utxGORIL0NMfb5B7xNJoZ+GFghMioXqWszbkmnV/CrunIYc1EYEx f4W78KdrarisPxECT8yVgmYORHwBk9bOsPKOWbXcERcCjRBBbSWxtq6cwft6xdqVLyIG 0XZUH2qKzDbmdvaKrgMIB82m1mQwFIzbfqnMrz7ucVMTXMvmoFYLHCSGHGD2LNJ3xben wE72wwOkIKQ45gt/Q6MoQL4evHDkiO3pTa/LdOYuR0sui+AuG6JPjl0w4ct+TXPuhcNM TvWQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=nl8/XcSo5npDTXvRYjKf28LcA8OJMzH5M7EJrj7FCrE=; b=hX1ZS9/YL/Biin9flndXHfWWXeq1Biurj+ayjVgmeMXJsqZ6aEyd9gjgA3e/wWnTsg DKfWzcumQyQ4o5iHQzF8aIss1GzZR8kqw649ZmNzGc4jkd2nyW8NRHsr3Z/htK1Z7PZK ohO8/4Uxs4UsKpnZNCQIoeG7POR7MjnbRh4YzW9ELSj0944SWyL1WW0WDcdxvzFZHRzN CPZxR834NYM3NgeFCOmOo58kh/HFm3XzfL1/H5vDV1Z80LIZPrP3BdiVw372vp7+Nu09 Oi/3Y63nqpmNGHv2LlIH7UkbYwG/WuNa0iR0XHR8NvKC8tijKUklCpA6/BGiZx7jFjGU D8rw== X-Gm-Message-State: AOAM531CC7plxoqs0kUERhkviIsicZ8AV4oM3UsnVocJTUKugjb4h4MZ wcOplozteouzEx1uC0EH1Sie0vF20STNJ1C+Dck= X-Google-Smtp-Source: ABdhPJynSSNGXgj8qUlEI1G+YZTJD3PV1ObCH4cbm6KUO684u+n7sV+CDebG6HE28cKAAwU5UXDx9VOaMcgPYaqS8xM= X-Received: by 2002:ac2:4d58:: with SMTP id 24mr872009lfp.32.1604578898187; Thu, 05 Nov 2020 04:21:38 -0800 (PST) MIME-Version: 1.0 References: <20201022122421.133976-1-gnurou@gmail.com> In-Reply-To: From: Alexandre Courbot Date: Thu, 5 Nov 2020 21:21:26 +0900 Message-ID: Subject: Re: [PATCH] media: v4l2-mem2mem: always call poll_wait() on queues To: Hans Verkuil Cc: Mauro Carvalho Chehab , linux-media , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 3, 2020 at 6:48 PM Hans Verkuil wrote: > > On 03/11/2020 09:51, Alexandre Courbot wrote: > > Hi Hans, > > > > On Sat, Oct 31, 2020 at 12:09 AM Hans Verkuil wrote: > >> > >> On 22/10/2020 14:24, Alexandre Courbot wrote: > >>> do_poll()/do_select() seem to set the _qproc member of poll_table to > >>> NULL the first time they are called on a given table, making subsequent > >>> calls of poll_wait() on that table no-ops. This is a problem for mem2mem > >>> which calls poll_wait() on the V4L2 queues' waitqueues only when a > >>> queue-related event is requested, which may not necessarily be the case > >>> during the first poll. > >>> > >>> For instance, a stateful decoder is typically only interested in > >>> EPOLLPRI events when it starts, and will switch to listening to both > >>> EPOLLPRI and EPOLLIN after receiving the initial resolution change event > >>> and configuring the CAPTURE queue. However by the time that switch > >>> happens and v4l2_m2m_poll_for_data() is called for the first time, > >>> poll_wait() has become a no-op and the V4L2 queues waitqueues thus > >>> cannot be registered. > >>> > >>> Fix this by moving the registration to v4l2_m2m_poll() and do it whether > >>> or not one of the queue-related events are requested. > >> > >> This looks good, but would it be possible to add a test for this to > >> v4l2-compliance? (Look for POLL_MODE_EPOLL in v4l2-test-buffers.cpp) > >> > >> If I understand this right, calling EPOLL_CTL_ADD for EPOLLPRI, then > >> calling EPOLL_CTL_ADD for EPOLLIN/OUT would trigger this? Or does there > >> have to be an epoll_wait call in between? > > > > Even without an epoll_wait() in between the behavior is visible. > > v4l2_m2m_poll() will be called once during the initial EPOLL_CTL_ADD > > and this will trigger the bug. > > > >> Another reason for adding this test is that I wonder if regular capture > >> or output V4L2 devices don't have the same issue. > >> > >> It's a very subtle bug and so adding a test for this to v4l2-compliance > >> would be very useful. > > > > I fully agree, this is very counter-intuitive since what basically > > happens is that the kernel's poll_wait() function becomes a no-op > > after the poll() hook of a driver is called for the first time. There > > is no way one can expect this behavior just from browsing the code so > > this is likely to affect other drivers. > > > > As for the test itself, we can easily reproduce the conditions for > > failure in v4l2-test-buffers.cpp's captureBufs() function, but doing > > so will make the streaming tests fail without being specific about the > > cause. Or maybe we should add another pollmode to specifically test > > epoll in this setup? Can I get your thoughts? > > No, just keep it as part of the poll test. Just add comments at the place > where it fails describing this error. > > After all, it *is* a poll() bug, so it is only fair that it is tested as > part of the epoll test. > > Can you call EPOLL_CTL_ADD with ev.events set to 0? And then call it again > with the actual value that you need? If that triggers this issue as well, > then that is a nice test (but perhaps EPOLL_CTL_ADD won't call poll() if > ev.events is 0, but perhaps EPOLLERR would work instead of 0). Yup, actually the following is enough to make v4l2-compliance -s fail with vicodec: diff --git a/utils/v4l2-compliance/v4l2-test-buffers.cpp b/utils/v4l2-compliance/v4l2-test-buffers.cpp index 8000db23..b63326cd 100644 --- a/utils/v4l2-compliance/v4l2-test-buffers.cpp +++ b/utils/v4l2-compliance/v4l2-test-buffers.cpp @@ -903,6 +903,10 @@ static int captureBufs(struct node *node, struct node *node_m2m_cap, const cv4l_ epollfd = epoll_create1(0); fail_on_test(epollfd < 0); + + ev.events = 0; + fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_ADD, node->g_fd(), &ev)); + if (node->is_m2m) ev.events = EPOLLIN | EPOLLOUT | EPOLLPRI; else if (v4l_type_is_output(q.g_type())) @@ -910,7 +914,7 @@ static int captureBufs(struct node *node, struct node *node_m2m_cap, const cv4l_ else ev.events = EPOLLIN; ev.data.fd = node->g_fd(); - fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_ADD, node->g_fd(), &ev)); + fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_MOD, node->g_fd(), &ev)); } if (pollmode) > > The epoll_wait() will fail when this issue hits, so that's a good place > to add comments explaining this problem. > > There is one other place where this needs to be tested: testEvents() in > v4l2-test-controls.cpp: currently this only tests select(), but there > should be a second epoll test here as well that just tests EPOLLPRI. > > This would catch drivers that do not stream (i.e. no EPOLLIN/OUT) but > that do have controls (so support EPOLLPRI). I'll take a look there as well, and think about a proper comment before sending a patch towards you. Cheers, Alex.