From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754176Ab1AUPd4 (ORCPT ); Fri, 21 Jan 2011 10:33:56 -0500 Received: from canuck.infradead.org ([134.117.69.58]:46598 "EHLO canuck.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753719Ab1AUPdx convert rfc822-to-8bit (ORCPT ); Fri, 21 Jan 2011 10:33:53 -0500 Subject: Re: [PATCH 00/21] mm: Preemptibility -v6 From: Peter Zijlstra To: Hugh Dickins Cc: Andrew Morton , Benjamin Herrenschmidt , David Miller , Nick Piggin , Martin Schwidefsky , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, Andrea Arcangeli , Oleg Nesterov , "Paul E. McKenney" In-Reply-To: References: <20101126143843.801484792@chello.nl> <1295457039.28776.137.camel@laptop> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Fri, 21 Jan 2011 16:33:54 +0100 Message-ID: <1295624034.28776.303.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2011-01-20 at 11:57 -0800, Hugh Dickins wrote: > > > 21/21 mm-optimize_page_lock_anon_vma_fast-path.patch > > > I certainly see the call for this patch, I want to eliminate those > > > doubled atomics too. This appears correct to me, and I've not dreamt > > > up an alternative; but I do dislike it, and I suspect you don't like > > > it much either. I'm ambivalent about it, would love a better patch. > > > > Like said, I fully agree with that sentiment, just haven't been able to > > come up with anything saner :/ Although I can optimize the > > __put_anon_vma() path a bit by doing something like: > > > > if (mutex_is_locked()) { anon_vma_lock(); anon_vma_unlock(); } > > > > But I bet that wants a barrier someplace and my head hurts.. > > Without daring to hurt my head very much, yes, I'd say those kind > of "optimizations" have a habit of turning out to be racily wrong. > > But you put your finger on it: if you hadn't had to add that lock- > unlock pair into __put_anon_vma(), I wouldn't have minded the > contortions added to page_lock_anon_vma(). I think there's just about enough implied barriers there that the 'simple' code just works ;-) But given that I'm trying to think with snot for brains thanks to some cold, I don't trust myself at all to have gotten this right. [ for Oleg and Paul: https://lkml.org/lkml/2010/11/26/213 contains the full patch this is against ] --- Index: linux-2.6/mm/rmap.c =================================================================== --- linux-2.6.orig/mm/rmap.c +++ linux-2.6/mm/rmap.c @@ -1559,9 +1559,20 @@ void __put_anon_vma(struct anon_vma *ano * Synchronize against page_lock_anon_vma() such that * we can safely hold the lock without the anon_vma getting * freed. + * + * Relies on the full mb implied by the atomic_dec_and_test() from + * put_anon_vma() against the full mb implied by mutex_trylock() from + * page_lock_anon_vma(). This orders: + * + * page_lock_anon_vma() VS put_anon_vma() + * mutex_trylock() atomic_dec_and_test() + * smp_mb() smp_mb() + * atomic_read() mutex_is_locked() */ - anon_vma_lock(anon_vma); - anon_vma_unlock(anon_vma); + if (mutex_is_locked(&anon_vma->root->mutex)) { + anon_vma_lock(anon_vma); + anon_vma_unlock(anon_vma); + } if (anon_vma->root != anon_vma) put_anon_vma(anon_vma->root);