From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id E483F6B003D for ; Wed, 18 Mar 2009 18:41:04 -0400 (EDT) Message-ID: <49C17880.7080109@redhat.com> Date: Thu, 19 Mar 2009 00:41:04 +0200 From: Avi Kivity MIME-Version: 1.0 Subject: Re: Question about x86/mm/gup.c's use of disabled interrupts References: <49C148AF.5050601@goop.org> <49C16411.2040705@redhat.com> <49C1665A.4080707@goop.org> <49C16A48.4090303@redhat.com> <49C17230.20109@goop.org> In-Reply-To: <49C17230.20109@goop.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Jeremy Fitzhardinge Cc: Nick Piggin , Linux Kernel Mailing List , Linux Memory Management List , Xen-devel , Jan Beulich , Ingo Molnar List-ID: Jeremy Fitzhardinge wrote: >> I thought you were concerned about cpu 0 doing a gup_fast(), cpu 1 >> doing P->N, and cpu 2 doing N->P. In this case cpu 2 is waiting on >> the pte lock. > > The issue is that if cpu 0 is doing a gup_fast() and other cpus are > doing P->P updates, then gup_fast() can potentially get a mix of old > and new pte values - where P->P is any aggregate set of unsynchronized > P->N and N->P operations on any number of other cpus. Ah, but if > every P->N is followed by a tlb flush, then disabling interrupts will > hold off any following N->P, allowing gup_fast to get a consistent pte > snapshot. > Right. > Hm, awkward if flush_tlb_others doesn't IPI... > How can it avoid flushing the tlb on cpu [01]? It's it's gup_fast()ing a pte, it may as well load it into the tlb. > > Simplest fix is to make gup_get_pte() a pvop, but that does seem like > putting a red flag in front of an inner-loop hotspot, or something... > > The per-cpu tlb-flush exclusion flag might really be the way to go. I don't see how it will work, without changing Xen to look at the flag? local_irq_disable() is used here to lock out a remote cpu, I don't see why deferring the flush helps. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org