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McKenney" To: Sasha Levin Cc: Mathieu Desnoyers , linux-kernel , Ingo Molnar , Lai Jiangshan , dipankar , Andrew Morton , Josh Triplett , Thomas Gleixner , Peter Zijlstra , rostedt , David Howells , Eric Dumazet , fweisbec , Oleg Nesterov , "Joel Fernandes, Google" , kernel-team Subject: Re: [PATCH tip/core/rcu 0/41] More RCU flavor consolidation cleanup for v4.21/v5.0 Reply-To: paulmck@linux.ibm.com References: <20181111194104.GA4787@linux.ibm.com> <2113574714.3779.1542031670315.JavaMail.zimbra@efficios.com> <20181112160137.GI4170@linux.ibm.com> <20181112214023.GA64029@sasha-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181112214023.GA64029@sasha-vm> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18111222-0072-0000-0000-000003C79C1C X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010037; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01116542; UDB=6.00577118; IPR=6.00896650; MB=3.00024134; MTD=3.00000008; XFM=3.00000015; UTC=2018-11-12 22:16:43 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18111222-0073-0000-0000-00004A1645B1 Message-Id: <20181112221635.GV4170@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-11-12_14:,, 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=8.0.1-1807170000 definitions=main-1811120190 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 12, 2018 at 04:40:23PM -0500, Sasha Levin wrote: > On Mon, Nov 12, 2018 at 08:01:37AM -0800, Paul E. McKenney wrote: > >On Mon, Nov 12, 2018 at 09:07:50AM -0500, Mathieu Desnoyers wrote: > >>----- On Nov 11, 2018, at 2:41 PM, paulmck paulmck@linux.ibm.com wrote: > >> > >>> Hello! > >>> > >>> This series does additional cleanup for the RCU flavor consolidation, > >>> focusing primarily on uses of old API members, for example, so that > >>> call_rcu_bh() becomes call_rcu(). There are also a few straggling > >>> internal-to-RCU cleanups. > >>> > >>> 1. Remove unused rcu_state externs, courtesy of Joel Fernandes. > >>> > >>> 2. Fix rcu_{node,data} comments about gp_seq_needed, courtesy of > >>> Joel Fernandes. > >>> > >>> 3. Eliminate synchronize_rcu_mult() and its sole caller. > >>> > >>> 4. Consolidate the RCU update functions invoked by sync.c. > >>> > >>> 5-41. Replace old flavorful RCU API calls with the corresponding > >>> vanilla calls. > >> > >>Hi Paul, > >> > >>Just a heads up: we might want to spell out warnings in very big letters > >>for anyone trying to backport code using RCU from post-4.21 kernels > >>back to older kernels. I fear that newer code will build just fine > >>on older kernels, but will spectacularly fail in hard-to-debug ways at > >>runtime. > >> > >>Renaming synchronize_rcu() and call_rcu() to something that did not > >>exist in prior kernels would prevent that. It may not be as pretty > >>though. > > > >From v4.20 rather than v4.21, but yes. Would it make sense to have Sasha > >automatically flag -stable candidates going back past that boundary that > >contain call_rcu(), synchronize_rcu(), etc.? Adding Sasha on CC, and > >I might be able to touch base with him this week. > > We had a similar issue recently with a vfs change > (https://patchwork.kernel.org/patch/10604339/) leading to potentially > the same results as described above, we took it as is to avoid these > issues in the future, though this is a much smaller change than what's > proposed here. > > We can look into an good way to solve this. While I can alert on > post-4.20 stable tagged patches that touch rcu, do you really want to be > dealing with this for the next 10+ years? It'll also means each of those > patches will need a manual backport. > > Let's talk at Plumbers :) Sounds like a plan! ;-) Thanx, Paul