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=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,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 99246C2D0EF for ; Fri, 17 Apr 2020 18:26:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7D12A20771 for ; Fri, 17 Apr 2020 18:26:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="pSVSKbwJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730609AbgDQS0o (ORCPT ); Fri, 17 Apr 2020 14:26:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43988 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1730407AbgDQS0o (ORCPT ); Fri, 17 Apr 2020 14:26:44 -0400 Received: from mail-qk1-x741.google.com (mail-qk1-x741.google.com [IPv6:2607:f8b0:4864:20::741]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D43E0C061A0C for ; Fri, 17 Apr 2020 11:26:43 -0700 (PDT) Received: by mail-qk1-x741.google.com with SMTP id o19so3456769qkk.5 for ; Fri, 17 Apr 2020 11:26:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=EGV5iBCdgUI9bxOhbrbPqcJYXUWmv8zZdPW9n0KUVqI=; b=pSVSKbwJ6KZDupYAQJM8TBEpDpoxgWL3SDWvOufXXp2TU8fEG92Ygsg5Br/CqHDPH5 2X+UyktOQtZwLOYyE75bqRZLY+qHcdrBmFhlLpWUs5Q7i6ndN2U4G8FJnT+ZTvmOa76P dLOPB+hPXx5w72eztEimFR1IzxoUBq4F+XXBI= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=EGV5iBCdgUI9bxOhbrbPqcJYXUWmv8zZdPW9n0KUVqI=; b=YQHsfPsyI+5hhtENIFU8hRyPbOvxXuesZ2xdZbnlJozBxlpmkqBq/UmRWAIZ9GI0qa knnFXo4l1oYk7Dw+KvGQbXI/XEc0wB7YVjBYZ3F9cM5tOcvjMyeE/waimi3t5iaJMLbh Gkv3FwkB/bjLyRFBwlEin8uRF9LySRcqO1L8COs/LJbVGKUC3KiuYJSrwDf3MdWPQAF3 1J7ooX0CdB17uJ+MWFwACIxiJba6DP2xqjBI9NKs+OmMnlvEaUFN5dIQoHragIwRI6Lo B75On0ZTZGeQt91XBtLJCLNVATgst6yZkUHxldJFAXWxxHJOrndwdldNB1fv8rhxczq2 xLvQ== X-Gm-Message-State: AGi0PuZOns1EzFf5ZyRVBxhxwlJL9PJSPdLA/yAfnYsFvy4tld+m7YHP z0lsnA1ZWA51Hb/UzgUeEe7DNg== X-Google-Smtp-Source: APiQypIhCpWG0cLNE8U8FY33oyf2Je0AVv7b/LPlQuj5JKl6ZzkGpA60u41QIx/FIhpy7KGSxJNpYA== X-Received: by 2002:a37:5347:: with SMTP id h68mr4643570qkb.451.1587148002869; Fri, 17 Apr 2020 11:26:42 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id b13sm14130361qka.20.2020.04.17.11.26.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 17 Apr 2020 11:26:42 -0700 (PDT) Date: Fri, 17 Apr 2020 14:26:41 -0400 From: Joel Fernandes To: Sebastian Andrzej Siewior Cc: "Paul E. McKenney" , Steven Rostedt , rcu@vger.kernel.org, Josh Triplett , Mathieu Desnoyers , Lai Jiangshan , Thomas Gleixner , Mike Galbraith , urezki@gmail.com Subject: Re: [PATCH 1/3] rcu: Use static initializer for krc.lock Message-ID: <20200417182641.GB168907@google.com> References: <20200416144254.GC90777@google.com> <20200416151824.a372pdiphube3x3l@linutronix.de> <20200416184112.GA149999@google.com> <20200416185934.GD149999@google.com> <20200416152623.48125628@gandalf.local.home> <20200416203637.GA176663@google.com> <20200416210057.GY17661@paulmck-ThinkPad-P72> <20200416213444.4cc6kzxmwl32s2eh@linutronix.de> <20200417030515.GE176663@google.com> <20200417150442.gyrxhjymvfwsvum5@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200417150442.gyrxhjymvfwsvum5@linutronix.de> Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Fri, Apr 17, 2020 at 05:04:42PM +0200, Sebastian Andrzej Siewior wrote: > On 2020-04-16 23:05:15 [-0400], Joel Fernandes wrote: > > On Thu, Apr 16, 2020 at 11:34:44PM +0200, Sebastian Andrzej Siewior wrote: > > > On 2020-04-16 14:00:57 [-0700], Paul E. McKenney wrote: > > > > > > > > We might need different calling-context restrictions for the two variants > > > > of kfree_rcu(). And we might need to come up with some sort of lockdep > > > > check for "safe to use normal spinlock in -rt". > > > > > > Oh. We do have this already, it is called CONFIG_PROVE_RAW_LOCK_NESTING. > > > This one will scream if you do > > > raw_spin_lock(); > > > spin_lock(); > > > > > > Sadly, as of today, there is code triggering this which needs to be > > > addressed first (but it is one list of things to do). > > > > > > Given the thread so far, is it okay if I repost the series with > > > migrate_disable() instead of accepting a possible migration before > > > grabbing the lock? I would prefer to avoid the extra RT case (avoiding > > > memory allocations in a possible atomic context) until we get there. > > > > I prefer something like the following to make it possible to invoke > > kfree_rcu() from atomic context considering call_rcu() is already callable > > from such contexts. Thoughts? > > So it looks like it would work. However, could we please delay this > until we have an actual case on RT? I just added > WARN_ON(!preemptible()); I am not sure if waiting for it to break in the future is a good idea. I'd rather design it in a forward thinking way. There could be folks replacing "call_rcu() + kfree in a callback" with kfree_rcu() for example. If they were in !preemptible(), we'd break on page allocation. Also as a sidenote, the additional pre-allocation of pages that Vlad is planning on adding would further reduce the need for pages from the page allocator. Paul, what is your opinion on this? thanks, - Joel > > to kfree_call_rcu() on v5.6.4-rt4 and nothing triggered. > > This is the list of users I had (just to figure out if this is used at > all): > - addrconf_ifdown > - cgroup_free > - cgroup_migrate_finish > - css_task_iter_end > - disk_expand_part_tbl > - drop_sysctl_table > - __hw_addr_flush > - inetdev_event > - ip6addrlbl_net_exit > - ip6addrlbl_net_exit > - ops_exit_list.isra.0 > - rtnl_register_internal > - simple_set_acl > - swevent_hlist_put_cpu > - timerfd_release > - vfs_rename > - __xfs_set_acl > > Sebastian