From: Jeremy Fitzhardinge Subject: Re: Question about x86/mm/gup.c's use of disabled interrupts Date: Wed, 18 Mar 2009 15:14:08 -0700 Message-ID: <49C17230.20109@goop.org> References: <49C148AF.5050601@goop.org> <49C16411.2040705@redhat.com> <49C1665A.4080707@goop.org> <49C16A48.4090303@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <49C16A48.4090303@redhat.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Avi Kivity Cc: Nick Piggin , Xen-devel , Linux Kernel Mailing List , Linux Memory Management List , Ingo Molnar List-Id: xen-devel@lists.xenproject.org Avi Kivity wrote: > Jeremy Fitzhardinge wrote: >>>> Disabling the interrupt will prevent the tlb flush IPI from coming >>>> in and flushing this cpu's tlb, but I don't see how it will prevent >>>> some other cpu from actually updating the pte in the pagetable, >>>> which is what we're concerned about here. >>> >>> The thread that cleared the pte holds the pte lock and is now >>> waiting for the IPI. The thread that wants to update the pte will >>> wait for the pte lock, thus also waits on the IPI and gup_fast()'s >>> local_irq_enable(). I think. >> >> But hasn't it already done the pte update at that point? >> >> (I think this conversation really is moot because the kernel never >> does P->P pte updates any more; its always P->N->P.) > > 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. Hm, awkward if flush_tlb_others doesn't IPI... > Won't stop munmap(). And I guess it does the tlb flush before freeing the pages, so disabling the interrupt helps here too. 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. J