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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 28 May 2019 13:24:43 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x4SCOgvP29491484 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 28 May 2019 12:24:43 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C2B1FB206E; Tue, 28 May 2019 12:24:42 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 994BFB2065; Tue, 28 May 2019 12:24:42 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.216]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Tue, 28 May 2019 12:24:42 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id C799516C35C0; Tue, 28 May 2019 05:24:47 -0700 (PDT) Date: Tue, 28 May 2019 05:24:47 -0700 From: "Paul E. McKenney" To: Joel Fernandes 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 Reply-To: paulmck@linux.ibm.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> <20190525181407.GA220326@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190525181407.GA220326@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19052812-0072-0000-0000-00000434284E X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011175; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01209781; UDB=6.00635551; IPR=6.00990810; MB=3.00027084; MTD=3.00000008; XFM=3.00000015; UTC=2019-05-28 12:24:48 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19052812-0073-0000-0000-00004C6504FA Message-Id: <20190528122447.GS28207@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-05-28_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1905280081 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 25, 2019 at 02:14:07PM -0400, Joel Fernandes wrote: > 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. Why not just rcu_dereference_protected(lockdep_is_held(some_mutex))? The API is already there, and no need for spurious readers. > > 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). Understood, but regardless of who suggested it, if we are to go through with it, good justification will be required. ;-) Thanx, Paul > thanks! > > - Joel > > > Thanx, Paul > > > > > thanks, Steven! > > > > > >