From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753062AbdEPMYF (ORCPT ); Tue, 16 May 2017 08:24:05 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:60303 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752749AbdEPMYA (ORCPT ); Tue, 16 May 2017 08:24:00 -0400 Date: Tue, 16 May 2017 05:23:54 -0700 From: "Paul E. McKenney" To: Ingo Molnar Cc: rostedt@goodmis.org, linux-kernel@vger.kernel.org Subject: Re: Use case for TASKS_RCU Reply-To: paulmck@linux.vnet.ibm.com References: <20170515182354.GA25440@linux.vnet.ibm.com> <20170516062233.tyz7ze7ilmbkxtjc@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170516062233.tyz7ze7ilmbkxtjc@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17051612-0024-0000-0000-00000270065C X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007071; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000212; SDB=6.00861252; UDB=6.00427148; IPR=6.00640916; BA=6.00005351; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015480; XFM=3.00000015; UTC=2017-05-16 12:23:56 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17051612-0025-0000-0000-000044042D32 Message-Id: <20170516122354.GB3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-05-16_03:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1705160103 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 16, 2017 at 08:22:33AM +0200, Ingo Molnar wrote: > > * Paul E. McKenney wrote: > > > Hello! > > > > The question of the use case for TASKS_RCU came up, and here is my > > understanding. Steve will not be shy about correcting any misconceptions > > I might have. ;-) > > > > The use case is to support freeing of trampolines used in tracing/probing > > in CONFIG_PREEMPT=y kernels. It is necessary to wait until any task > > executing in the trampoline in question has left it, taking into account > > that the trampoline's code might be interrupted and preempted. However, > > the code in the trampolines is guaranteed never to context switch. > > > > Note that in CONFIG_PREEMPT=n kernels, synchronize_sched() suffices. > > It is therefore tempting to think in terms of disabling preemption across > > the trampolines, but there is apparently not enough room to accommodate > > the needed preempt_disable() and preempt_enable() in the code invoking > > the trampoline, and putting the preempt_disable() and preempt_enable() > > in the trampoline itself fails because of the possibility of preemption > > just before the preempt_disable() and just after the preempt_enable(). > > Similar reasoning rules out use of rcu_read_lock() and rcu_read_unlock(). > > So how was this solved before TASKS_RCU? Also, nothing uses call_rcu_tasks() at > the moment, so it's hard for me to review its users. What am I missing? Before TASKS_RCU, the trampolines were just leaked when CONFIG_PREEMPT=y. Current mainline kernel/trace/ftrace.c uses synchronize_rcu_tasks(). So yes, currently one user. Thanx, Paul