From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756414AbaHHRJX (ORCPT ); Fri, 8 Aug 2014 13:09:23 -0400 Received: from e32.co.us.ibm.com ([32.97.110.150]:58515 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752812AbaHHRJW (ORCPT ); Fri, 8 Aug 2014 13:09:22 -0400 Date: Fri, 8 Aug 2014 10:09:16 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Steven Rostedt , Oleg Nesterov , linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, bobby.prani@gmail.com, masami.hiramatsu.pt@hitachi.com Subject: Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period waiting for RCU-tasks Message-ID: <20140808170916.GB5821@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140807155326.18481e66@gandalf.local.home> <20140807200813.GB3935@laptop> <20140807171823.1a481290@gandalf.local.home> <20140808064020.GZ9918@twins.programming.kicks-ass.net> <20140808101221.21056900@gandalf.local.home> <20140808143413.GB9918@twins.programming.kicks-ass.net> <20140808105858.171da847@gandalf.local.home> <20140808162714.GG9918@twins.programming.kicks-ass.net> <20140808163905.GA5821@linux.vnet.ibm.com> <20140808165112.GI9918@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140808165112.GI9918@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14080817-0928-0000-0000-000003FAC1BA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 08, 2014 at 06:51:12PM +0200, Peter Zijlstra wrote: > On Fri, Aug 08, 2014 at 09:39:05AM -0700, Paul E. McKenney wrote: > > On Fri, Aug 08, 2014 at 06:27:14PM +0200, Peter Zijlstra wrote: > > > On Fri, Aug 08, 2014 at 10:58:58AM -0400, Steven Rostedt wrote: > > > > > > > > > No, they are also used by optimized kprobes. This is why optimized > > > > > > kprobes depend on !CONFIG_PREEMPT. [ added Masami to the discussion ]. > > > > > > > > > > How do those work? Is that one where the INT3 relocates the instruction > > > > > stream into an alternative 'text' and that JMPs back into the original > > > > > stream at the end? > > > > > > > > No, it's where we replace the 'int3' with a jump to a trampoline that > > > > simulates an INT3. Speeds things up quite a bit. > > > > > > OK, so the trivial 'fix' for that is to patch the probe site like: > > > > > > preempt_disable(); INC GS:%__preempt_count > > > call trampoline; CALL 0xDEADBEEF > > > preempt_enable(); DEC GS:%__preempt_count > > > JNZ 1f > > > CALL ___preempt_schedule > > > 1f: > > > > > > At which point the preempt_disable/enable() are the read side primitives > > > and call_rcu_sched/synchronize_sched are sufficient to release it. > > > > Unless this is done in idle, at which point RCU-sched is studiously > > ignoring any preempt_disable() sections. > > Well, given that kprobes is already using it, it 'must' be good ;-) I > suspect much of the idle loop is marked with __kprobe or so, or nobody > has been brave enough to try. Not seeing much in the way of __kprobe, so guessing lack of bravery. Thanx, Paul