From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-6.1 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI,SPF_HELO_NONE,SPF_NONE autolearn=unavailable autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 29D247D981 for ; Sat, 25 May 2019 18:14:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727578AbfEYSOL (ORCPT ); Sat, 25 May 2019 14:14:11 -0400 Received: from mail-pg1-f195.google.com ([209.85.215.195]:40665 "EHLO mail-pg1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727562AbfEYSOL (ORCPT ); Sat, 25 May 2019 14:14:11 -0400 Received: by mail-pg1-f195.google.com with SMTP id d30so6802592pgm.7 for ; Sat, 25 May 2019 11:14:10 -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:user-agent; bh=WMR94oFTY+OqH2rjdVLemgAhQSEwpDtMoTvHog5j0sQ=; b=X0CnhJKSN5RJR4iBBE0j8IyN20bJySbgLRYLCC51oQloSv5szaUMYCNZpmOF7YbrDu djpT8FbnXBj3XQkIAMR6RR9RpzFcWEVC3LhAMyeOqLUzZjvP8fjtU34iUpZcXld1un1M b02atJSdTlxwLlu/aDOH5x6Q5ekm4aDR1+1vE= 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:user-agent; bh=WMR94oFTY+OqH2rjdVLemgAhQSEwpDtMoTvHog5j0sQ=; b=d71QsUi3tlonmZryc5lIlqLMxrSVIwDLFsL4eLDcpqevz29ZfpRhpm+nIzpp6pEYDp fS4lQFi30W5wNCEQNZ1wKu6/L5X54Qvee3LmlVgHIXKE2vmJQ1EeNGPXeI8GyXVeuIh+ KIwV2IaKjHtOaSLNSDnwMi47ztdd5wVczOPF2//GJWwiPT9EgO+rV35wwT4Fg8KbMUEK hhZA7Ay8gYeNuJnklXkqveB+ZTLqIJKrAS4XSggnVqwmCWZRFeV43n26kNrM47SIse2T yKoFAZtQ5cW2GEgU1F0lSo7UVnygFjh3Mo+xyXaSlGbIvzNfBJzd1QcaySBtsdT/vzd6 gG0Q== X-Gm-Message-State: APjAAAWU1VR/eqcpQGQpjmtVswEVVX4LmZQfRisKxDZ6FSRpU+F0j7Fc ZBKJE7PlR8S0V8bfIDwEqTbV0Q== X-Google-Smtp-Source: APXvYqyQRiZeJdnv8NV0jJQ1QNMV+NOKlT6Luw5GwD0AE0gpx9GAm4B91udlCkCjw5QI0crgiiIVzA== X-Received: by 2002:a63:4710:: with SMTP id u16mr11132086pga.447.1558808050038; Sat, 25 May 2019 11:14:10 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id y13sm8409814pfb.143.2019.05.25.11.14.08 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sat, 25 May 2019 11:14:08 -0700 (PDT) Date: Sat, 25 May 2019 14:14:07 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: Steven Rostedt , linux-kernel@vger.kernel.org, Benjamin Herrenschmidt , Ingo Molnar , Jonathan Corbet , Josh Triplett , kvm-ppc@vger.kernel.org, Lai Jiangshan , linux-doc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Mathieu Desnoyers , Michael Ellerman , Miguel Ojeda , Paul Mackerras , rcu@vger.kernel.org Subject: Re: [PATCH RFC 0/5] Remove some notrace RCU APIs Message-ID: <20190525181407.GA220326@google.com> References: <20190524234933.5133-1-joel@joelfernandes.org> <20190524232458.4bcf4eb4@gandalf.local.home> <20190525081444.GC197789@google.com> <20190525070826.16f76ee7@gandalf.local.home> <20190525141954.GA176647@google.com> <20190525155035.GE28207@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190525155035.GE28207@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Sat, May 25, 2019 at 08:50:35AM -0700, Paul E. McKenney wrote: > On Sat, May 25, 2019 at 10:19:54AM -0400, Joel Fernandes wrote: > > On Sat, May 25, 2019 at 07:08:26AM -0400, Steven Rostedt wrote: > > > On Sat, 25 May 2019 04:14:44 -0400 > > > Joel Fernandes wrote: > > > > > > > > I guess the difference between the _raw_notrace and just _raw variants > > > > > is that _notrace ones do a rcu_check_sparse(). Don't we want to keep > > > > > that check? > > > > > > > > This is true. > > > > > > > > Since the users of _raw_notrace are very few, is it worth keeping this API > > > > just for sparse checking? The API naming is also confusing. I was expecting > > > > _raw_notrace to do fewer checks than _raw, instead of more. Honestly, I just > > > > want to nuke _raw_notrace as done in this series and later we can introduce a > > > > sparse checking version of _raw if need-be. The other option could be to > > > > always do sparse checking for _raw however that used to be the case and got > > > > changed in http://lists.infradead.org/pipermail/linux-afs/2016-July/001016.html > > > > > > What if we just rename _raw to _raw_nocheck, and _raw_notrace to _raw ? > > > > That would also mean changing 160 usages of _raw to _raw_nocheck in the > > kernel :-/. > > > > The tracing usage of _raw_notrace is only like 2 or 3 users. Can we just call > > rcu_check_sparse directly in the calling code for those and eliminate the APIs? > > > > I wonder what Paul thinks about the matter as well. > > My thought is that it is likely that a goodly number of the current uses > of _raw should really be some form of _check, with lockdep expressions > spelled out. Not that working out what exactly those lockdep expressions > should be is necessarily a trivial undertaking. ;-) Yes, currently where I am a bit stuck is the rcu_dereference_raw() cannot possibly know what SRCU domain it is under, so lockdep cannot check if an SRCU lock is held without the user also passing along the SRCU domain. I am trying to change lockdep to see if it can check if *any* srcu domain lock is held (regardless of which one) and complain if none are. This is at least better than no check at all. However, I think it gets tricky for mutexes. If you have something like: mutex_lock(some_mutex); p = rcu_dereference_raw(gp); mutex_unlock(some_mutex); This might be a perfectly valid invocation of _raw, however my checks (patch is still cooking) trigger a lockdep warning becase _raw cannot know that this is Ok. lockdep thinks it is not in a reader section. This then gets into the territory of a new rcu_derference_raw_protected(gp, assert_held(some_mutex)) which sucks because its yet another API. To circumvent this issue, can we just have callers of rcu_dereference_raw ensure that they call rcu_read_lock() if they are protecting dereferences by a mutex? That would make things a lot easier and also may be Ok since rcu_read_lock is quite cheap. > That aside, if we are going to change the name of an API that is > used 160 places throughout the tree, we would need to have a pretty > good justification. Without such a justification, it will just look > like pointless churn to the various developers and maintainers on the > receiving end of the patches. Actually, the API name change is not something I want to do, it is Steven suggestion. My suggestion is let us just delete _raw_notrace and just use the _raw API for tracing, since _raw doesn't do any tracing anyway. Steve pointed that _raw_notrace does sparse checking unlike _raw, but I think that isn't an issue since _raw doesn't do such checking at the moment anyway.. (if possible check my cover letter again for details/motivation of this series). thanks! - Joel > Thanx, Paul > > > thanks, Steven! > > >