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=-8.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 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 8B6B6C4742C for ; Thu, 5 Nov 2020 12:36:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1CE712083B for ; Thu, 5 Nov 2020 12:36:54 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=xs4all.nl header.i=@xs4all.nl header.b="ksi+PJfI" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730714AbgKEMgx (ORCPT ); Thu, 5 Nov 2020 07:36:53 -0500 Received: from lb3-smtp-cloud8.xs4all.net ([194.109.24.29]:54561 "EHLO lb3-smtp-cloud8.xs4all.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728371AbgKEMgl (ORCPT ); Thu, 5 Nov 2020 07:36:41 -0500 Received: from cust-b5b5937f ([IPv6:fc0c:c16d:66b8:757f:c639:739b:9d66:799d]) by smtp-cloud8.xs4all.net with ESMTPA id aeVGkCvLkNanzaeVJkoKjC; Thu, 05 Nov 2020 13:36:38 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=xs4all.nl; s=s1; t=1604579798; bh=P4E26n1MJrzrkJ2BRBbM1Fj1NgVEkyFGFQpgoUFh2PY=; h=Subject:To:From:Message-ID:Date:MIME-Version:Content-Type:From: Subject; b=ksi+PJfIxpSrYn9zCen1lqmNWkv8DlLrX3mYLqZST07O5M9uAzClmVwIJy30YVSXu mgOzjIiKDTXPvYIgsn+BHmqeBz61BTgyI/v35swQasMeoRSM1U+WUn13DECpiifmR8 LekXUM8t5LCoyGsT37zbvefPY44GfSsXU/D9+2vTbeKqEFrrLmOhGSzaUJ2PxC/ots 5bO5bLTeAyIygt/7X583A4DJjuz+RvDOntIgOAPh2IcHv4kLI+oOMdirPHCaFti0al Rp/cIocanYEYKqU/2klYJnBe0IOoueicOuvNI7GKd0+zD6fNcS6FXh9BZLq4b9F5Yl fPifmIcgadedQ== Subject: Re: [PATCH] media: v4l2-mem2mem: always call poll_wait() on queues To: Alexandre Courbot Cc: Mauro Carvalho Chehab , linux-media , Linux Kernel Mailing List References: <20201022122421.133976-1-gnurou@gmail.com> From: Hans Verkuil Message-ID: <695e6163-7bdc-d120-cd02-0cff6efb53ef@xs4all.nl> Date: Thu, 5 Nov 2020 13:36:34 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-CMAE-Envelope: MS4xfNcGXtHXNq/iZVASkE2z7dcJw88GClwRw+cZbTPA4ITww+IQdKxsGgvXk8ePFP7pec4E/8WllbIPQX1uK1maKD+m7vwT0n6BCsJ+zbw8s++7XpLWQeDb YsUj2cnVy53NCPa+7J2JzXC6ODpeiCCh0J1qWe/ACGeLV3+f/EmEEV3P+em09NHQlKtq7OFVIJjaUrvEzSpFoY92/WT0u5nv5RD2lGyxP0gKaSaOBJ5vZpxJ 2KdEDee+Xyavl3VcWmJVOsHc6V5/LhkOaCshheV/hlhE8hdYvZ0ZNSX/uvUG3gMkgyGZg4tD16LM9zDhmrdI7A== Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/11/2020 13:21, Alexandre Courbot wrote: > 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: Does it also fail with vivid? I am curious to know whether this issue is m2m specific or a more general problem. Regards, Hans > > 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. >