From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757203Ab0DAP4R (ORCPT ); Thu, 1 Apr 2010 11:56:17 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:57509 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757049Ab0DAP4H (ORCPT ); Thu, 1 Apr 2010 11:56:07 -0400 Subject: Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in mmu_take_all_locks() From: Peter Zijlstra To: Andrea Arcangeli Cc: Avi Kivity , Thomas Gleixner , Rik van Riel , linux-kernel@vger.kernel.org, akpm@linux-foundation.org, Kent Overstreet , Ingo Molnar In-Reply-To: <1270137002.1598.65.camel@laptop> References: <1270117906.1653.139.camel@laptop> <4BB47FC3.1020606@redhat.com> <4BB480CC.2060503@redhat.com> <1270121264.1653.205.camel@laptop> <1270122194.1653.223.camel@laptop> <20100401154249.GQ5825@random.random> <1270137002.1598.65.camel@laptop> Content-Type: text/plain; charset="UTF-8" Date: Thu, 01 Apr 2010 17:56:02 +0200 Message-ID: <1270137362.1598.77.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2010-04-01 at 17:50 +0200, Peter Zijlstra wrote: > On Thu, 2010-04-01 at 17:42 +0200, Andrea Arcangeli wrote: > > On Thu, Apr 01, 2010 at 01:43:14PM +0200, Peter Zijlstra wrote: > > > On Thu, 2010-04-01 at 13:27 +0200, Peter Zijlstra wrote: > > > > > > > > I've almost got a patch done that converts those two, still need to look > > > > where that tasklist_lock muck happens. > > > > > > OK, so the below builds and boots, only need to track down that > > > tasklist_lock nesting, but I got to run an errand first. > > > > You should have a look at my old patchset where Christoph already > > implemented this (and not for decreasing latency but to allow > > scheduling in mmu notifier handlers, only needed by XPMEM): > > > > http://www.kernel.org/pub/linux/kernel/people/andrea/patches/v2.6/2.6.26-rc7/mmu-notifier-v18/ > > > > The ugliest part of it (that I think you missed below) is the breakage > > of the RCU locking in the anon-vma which requires adding refcounting > > to it. That was the worst part of the conversion as far as I can tell. > > > > http://www.kernel.org/pub/linux/kernel/people/andrea/patches/v2.6/2.6.26-rc7/mmu-notifier-v18/anon-vma > > > > I personally prefer read-write locks that Christoph used for both of > > them, but I'm not against mutex either. Still the refcounting problem > > should be the same as it's introduced by allowing the critical > > sections under anon_vma->lock to schedule (no matter if it's mutex or > > read-write sem). > > Right, so the problem with the rwsem is that, esp for very short hold > times, they introduce more pain than they're worth. Also the rwsem > doesn't do adaptive spinning nor allows for lock stealing, resulting in > a much much heavier sync. object than the mutex is. > > You also seem to move the tlb_gather stuff around, we have patches in > -rt that make tlb_gather preemptible, once i_mmap_lock is preemptible we > can do in mainline too. Another thing is mm->nr_ptes, that doens't appear to be properly serialized, __pte_alloc() does ++ under mm->page_table_lock, but free_pte_range() does -- which afaict isn't always with page_table_lock held, it does however always seem to have mmap_sem for writing. However __pte_alloc() callers do not in fact hold mmap_sem for writing.