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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 60639C35DEA for ; Tue, 25 Feb 2020 00:38:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2D50921927 for ; Tue, 25 Feb 2020 00:38:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582591120; bh=jq3ODePJ3Q6zoN0OHkVIb2ToZ28VrtYjD3/dPpBAwUM=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=ALE/zX5KkJcXXJnxiAx/Fx43aojZDICRdqm/QN8+KGTquVUh8Ccl178hhoZhNfEXb gWnmYfSL21d+owzuy5fR2Kt85UIMLKgB1nAF63YdPyMoWi5/a7D248I9yDe0bY5mXS zX04UC3fsDEQOOobuolUPOrCdzzwaXMwsGpapfq0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728593AbgBYAij (ORCPT ); Mon, 24 Feb 2020 19:38:39 -0500 Received: from mail.kernel.org ([198.145.29.99]:43460 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728011AbgBYAii (ORCPT ); Mon, 24 Feb 2020 19:38:38 -0500 Received: from localhost.localdomain (c-73-231-172-41.hsd1.ca.comcast.net [73.231.172.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 727EC2072D; Tue, 25 Feb 2020 00:38:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582591116; bh=jq3ODePJ3Q6zoN0OHkVIb2ToZ28VrtYjD3/dPpBAwUM=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=0bkniVS3jwtRsC+8G6oMWVV5CiKekmBhS3eBDkSEHm1jAB9Mfu7zT2BAsRTwSnOI/ nif9ExSCKPT2copXKF7CCogu+iOq9LFdzn2HmqT3glMgLZcwi4tcTrNbdL8/+akbXD NpvxEngILyVqP2R1oZSb+x6GvzBiTtenwtM26xGI= Date: Mon, 24 Feb 2020 16:38:35 -0800 From: Andrew Morton To: Jason Baron Cc: dave@stgolabs.net, rpenyaev@suse.de, linux-kernel@vger.kernel.org, normalperson@yhbt.net, viro@zeniv.linux.org.uk Subject: Re: [PATCH] fs/epoll: make nesting accounting safe for -rt kernel Message-Id: <20200224163835.08ab964483519052d7c2e39b@linux-foundation.org> In-Reply-To: <1579288607-11868-1-git-send-email-jbaron@akamai.com> References: <20200106210104.4hqlgpujqujcbeg7@linux-p48b> <1579288607-11868-1-git-send-email-jbaron@akamai.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 17 Jan 2020 14:16:47 -0500 Jason Baron wrote: > Davidlohr Bueso pointed out that when CONFIG_DEBUG_LOCK_ALLOC is set > ep_poll_safewake() can take several non-raw spinlocks after disabling > pre-emption which is no no for the -rt kernel. So let's re-work how we > determine the nesting level such that it plays nicely with -rt kernel. "no no" isn't terribly informative, and knowledge of -rt's requirements isn't widespread. Can we please spell this requirement out in full detail, if only to spread the -rt education a bit? > Let's introduce a 'nests' field in struct eventpoll that records the > current nesting level during ep_poll_callback(). Then, if we nest again we > can find the previous struct eventpoll that we were called from and > increase our count by 1. The 'nests' field is protected by > ep->poll_wait.lock. > > I've also moved napi_id field into a hole in struct eventpoll as part of > introduing the nests field. This change reduces the struct eventpoll from > 184 bytes to 176 bytes on x86_64 for the !CONFIG_DEBUG_LOCK_ALLOC > production config. > > ... > > @@ -551,30 +557,43 @@ static int ep_call_nested(struct nested_calls *ncalls, > */ > #ifdef CONFIG_DEBUG_LOCK_ALLOC > > -static DEFINE_PER_CPU(int, wakeup_nest); > - > -static void ep_poll_safewake(wait_queue_head_t *wq) > +static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi) > { > + struct eventpoll *ep_src; > unsigned long flags; > - int subclass; > + u8 nests = 0; > > - local_irq_save(flags); > - preempt_disable(); > - subclass = __this_cpu_read(wakeup_nest); > - spin_lock_nested(&wq->lock, subclass + 1); > - __this_cpu_inc(wakeup_nest); > - wake_up_locked_poll(wq, POLLIN); > - __this_cpu_dec(wakeup_nest); > - spin_unlock(&wq->lock); > - local_irq_restore(flags); > - preempt_enable(); > + /* > + * If we are not being call from ep_poll_callback(), epi is > + * NULL and we are at the first level of nesting, 0. Otherwise, > + * we are being called from ep_poll_callback() and if a previous > + * wakeup source is not an epoll file itself, we are at depth > + * 1 since the wakeup source is depth 0. If the wakeup source > + * is a previous epoll file in the wakeup chain then we use its > + * nests value and record ours as nests + 1. The previous epoll > + * file nests value is stable since its already holding its > + * own poll_wait.lock. > + */ Similarly, it would be helpful if this comment were to explain that this code exists for -rt's requirements, and to briefly describe what that requirement is. > + if (epi) { > + if ((is_file_epoll(epi->ffd.file))) { > + ep_src = epi->ffd.file->private_data; > + nests = ep_src->nests; > + } else { > + nests = 1; > + } > + } > + spin_lock_irqsave_nested(&ep->poll_wait.lock, flags, nests); > + ep->nests = nests + 1; > + wake_up_locked_poll(&ep->poll_wait, EPOLLIN); > + ep->nests = 0; > + spin_unlock_irqrestore(&ep->poll_wait.lock, flags); > }