From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752892AbbJFSIB (ORCPT ); Tue, 6 Oct 2015 14:08:01 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:36271 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751531AbbJFSH7 (ORCPT ); Tue, 6 Oct 2015 14:07:59 -0400 X-IBM-Helo: d03dlp03.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Tue, 6 Oct 2015 11:07:34 -0700 From: "Paul E. McKenney" To: Josh Triplett Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of __srcu_read_lock() Message-ID: <20151006180734.GA28562@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20151006161305.GA9799@linux.vnet.ibm.com> <1444148028-11551-1-git-send-email-paulmck@linux.vnet.ibm.com> <1444148028-11551-7-git-send-email-paulmck@linux.vnet.ibm.com> <20151006171839.GD9600@cloud> <20151006173646.GJ3910@linux.vnet.ibm.com> <20151006174311.GA10272@cloud> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151006174311.GA10272@cloud> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15100618-8236-0000-0000-00001279C94D Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 06, 2015 at 10:43:11AM -0700, Josh Triplett wrote: > On Tue, Oct 06, 2015 at 10:36:46AM -0700, Paul E. McKenney wrote: > > On Tue, Oct 06, 2015 at 10:18:39AM -0700, Josh Triplett wrote: > > > On Tue, Oct 06, 2015 at 09:13:42AM -0700, Paul E. McKenney wrote: > > > > Currently, __srcu_read_lock() cannot be invoked from restricted > > > > environments because it contains calls to preempt_disable() and > > > > preempt_enable(), both of which can invoke lockdep, which is a bad > > > > idea in some restricted execution modes. This commit therefore moves > > > > the preempt_disable() and preempt_enable() from __srcu_read_lock() > > > > to srcu_read_lock(). It also inserts the preempt_disable() and > > > > preempt_enable() around the call to __srcu_read_lock() in do_exit(). > > > > > > What restricted environments do you intend to invoke > > > __srcu_read_lock from? > > > > > > This change seems fine, but I don't see any change in this patch series > > > that needs this, hence my curiosity. > > > > Someone asked me for it, and now I cannot find it. :-( > > > > Something to the effect of when running unmapped during exception entry > > or something like that. I guess one way to find out would be to remove > > the commit and see who complained, but on the other hand, it arguably > > makes more sense to have only the bare mechanism is __srcu_read_lock() > > and put the environmental protection into srcu_read_lock(). > > I agree; I just find the idea that someone would need to call > __srcu_read_lock rather than srcu_read_lock odd and worthy of further > understanding. :) And they did supply an explanation that seemed satisfactory at the time, but I cannot find that either. I clearly need to track that sort of stuff better! Thanx, Paul