Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752721Ab0AGUZb (ORCPT ); Thu, 7 Jan 2010 15:25:31 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752561Ab0AGUZa (ORCPT ); Thu, 7 Jan 2010 15:25:30 -0500 Received: from e3.ny.us.ibm.com ([32.97.182.143]:39079 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752230Ab0AGUZ3 (ORCPT ); Thu, 7 Jan 2010 15:25:29 -0500 Date: Thu, 7 Jan 2010 12:25:26 -0800 From: "Paul E. McKenney" To: Linus Torvalds Cc: Peter Zijlstra , Christoph Lameter , Arjan van de Ven , KAMEZAWA Hiroyuki , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "minchan.kim@gmail.com" , "hugh.dickins" , Nick Piggin , Ingo Molnar Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault() Message-ID: <20100107202526.GQ6764@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100105192243.1d6b2213@infradead.org> <1262884960.4049.106.camel@laptop> <20100107192035.GO6764@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1752 Lines: 47 On Thu, Jan 07, 2010 at 12:06:48PM -0800, Linus Torvalds wrote: > > > On Thu, 7 Jan 2010, Paul E. McKenney wrote: > > > + > > > + spin_lock(&mm->page_table_lock); > > > + if (vma->vm_end == cur_brk) { > > > + vma->vm_end = brk; > > > + mm->brk = brk; > > > + cur_brk = brk; > > > + } > > > + spin_unlock(&mm->page_table_lock); > > > + > > > + if (cur_brk != brk) > > > > Can this be "if (cur_brk < brk)"? Seems like it should, given the > > earlier tests, but I don't claim to understand the VM code. > > It's really just a flag, to test whether the final check (inside the > spinlock) succeeded, or whether we perhaps raced with _another_ brk() call > that also had the mm_sem for reading. > > We know that cur_brk was different from brk before - because otherwise > we'd have just returned early (or done the slow case). So testing whether > it's different afterwards really only tests whether that > > cur_brk = brk; > > statment was executed or not. > > I could have used a separate flag called "success" or something. I was (perhaps confusedly) thinking of a pair of threads both trying to sbrk() at the same time. One of them wins by acquiring the ->page_table_lock first. Then the other acquires the spinlock, but sees vma->vm_end != cur_brk. But if the first one extended the region at least as far as the second intended to, the second one's work is done. Of course, we can debate the sanity of an application that actually does concurrent sbrk() calls. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/