From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753350AbbJFRnR (ORCPT ); Tue, 6 Oct 2015 13:43:17 -0400 Received: from relay4-d.mail.gandi.net ([217.70.183.196]:56138 "EHLO relay4-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752909AbbJFRnQ (ORCPT ); Tue, 6 Oct 2015 13:43:16 -0400 Date: Tue, 6 Oct 2015 10:43:11 -0700 From: Josh Triplett To: "Paul E. McKenney" 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: <20151006174311.GA10272@cloud> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151006173646.GJ3910@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. :)