From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [BELATED CORE TOPIC] context tracking / nohz / RCU state Date: Wed, 12 Aug 2015 08:59:19 -0700 Message-ID: <20150812155919.GU3895@linux.vnet.ibm.com> References: <20150811183312.GE3895@linux.vnet.ibm.com> <20150812143819.GE21542@lerouge> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from e33.co.us.ibm.com ([32.97.110.151]:59117 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753477AbbHLP70 (ORCPT ); Wed, 12 Aug 2015 11:59:26 -0400 Received: from /spool/local by e33.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 12 Aug 2015 09:59:25 -0600 Content-Disposition: inline In-Reply-To: <20150812143819.GE21542@lerouge> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Frederic Weisbecker Cc: Andy Lutomirski , "ksummit-discuss@lists.linuxfoundation.org" , "linux-kernel@vger.kernel.org" , Christoph Lameter , Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Peter Zijlstra , Chris Metcalf , Rik van Riel , benh@kernel.crashing.org, will.deacon@arm.com, linux-arch@vger.kernel.org On Wed, Aug 12, 2015 at 04:38:21PM +0200, Frederic Weisbecker wrote: > On Tue, Aug 11, 2015 at 11:33:12AM -0700, Paul E. McKenney wrote: > > On Tue, Aug 11, 2015 at 10:49:36AM -0700, Andy Lutomirski wrote: > > > Some arches may need: > > > > > > i_am_lame_and_forgot_my_previous_context() > > > > > > x86 will soon (4.3 or 4.4, depending on how my syscall cleanup goes) > > > no longer need that. > > > > > > Paul says that some arches need something that goes straight from IRQ > > > to user mode (?) -- sigh. > > > > Straight from IRQ to process-level kernel mode. I ran into this in > > late 2011, and clearly should have documented exactly what code was > > doing this. Something about invoking system calls from within the > > kernel on some architectures. > > > > Hey, if no architectures do this anymore, I could simplify RCU a bit! ;-) > > That issue has always been a bit foggy to me :-) > > We never really stated what exactly the issue was. Just performing syscalls > from kernel mode shouldn't fiddle with the dynticks count. > > IIUC, the issue was that some IRQs triggered and never returned. But we > certainly can't remove the safety code without clearly identifying the > issue... This was not a theoretical problem -- there were real failures. But yes, the safety code is there and seems to work OK, so I do need confirmation of a change before removing it. I do recall someone arguing that the half-interrupts should go away, but I never did hear that they really did go away. Adding linux-arch in the hope that someone can say for sure. Thanx, Paul