From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [patch 2/2]: introduce fast_gup Date: Mon, 21 Apr 2008 15:00:04 +0300 Message-ID: <480C81C4.8030200@qumranet.com> References: <20080328025455.GA8083@wotan.suse.de> <20080328030023.GC8083@wotan.suse.de> <1208444605.7115.2.camel@twins> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-arch-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: To: Linus Torvalds Cc: Peter Zijlstra , Nick Piggin , Andrew Morton , shaggy-V7BBcbaFuwjMbYB6QlFGEg@public.gmane.org, axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-arch-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Clark Williams , Ingo Molnar Linus Torvalds wrote: > Finally, I don't think that comment is correct in the first place. It's > not that simple. The thing is, even *with* the memory barrier in place, we > may have: > > CPU#1 CPU#2 > ===== ===== > > fast_gup: > - read low word > > native_set_pte_present: > - set low word to 0 > - set high word to new value > > - read high word > > - set low word to new value > > and so you read a low word that is associated with a *different* high > word! Notice? > > So trivial memory ordering is _not_ enough. > > So I think the code literally needs to be something like this > > #ifdef CONFIG_X86_PAE > > static inline pte_t native_get_pte(pte_t *ptep) > { > pte_t pte; > > retry: > pte.pte_low = ptep->pte_low; > smp_rmb(); > pte.pte_high = ptep->pte_high; > smp_rmb(); > if (unlikely(pte.pte_low != ptep->pte_low) > goto retry; > return pte; > } > > I think this is still broken. Suppose that after reading pte_high native_set_pte() is called again on another cpu, changing pte_low back to the original value (but with a different pte_high). You now have pte_low from second native_set_pte() but pte_high from the first native_set_pte(). You could use cmpxchg8b to atomically load the pte, but at the expense of taking the cacheline for exclusive access. -- error compiling committee.c: too many arguments to function From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from bzq-179-150-194.static.bezeqint.net ([212.179.150.194]:10382 "EHLO il.qumranet.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755660AbYDUMAG (ORCPT ); Mon, 21 Apr 2008 08:00:06 -0400 Message-ID: <480C81C4.8030200@qumranet.com> Date: Mon, 21 Apr 2008 15:00:04 +0300 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [patch 2/2]: introduce fast_gup References: <20080328025455.GA8083@wotan.suse.de> <20080328030023.GC8083@wotan.suse.de> <1208444605.7115.2.camel@twins> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Linus Torvalds Cc: Peter Zijlstra , Nick Piggin , Andrew Morton , shaggy@austin.ibm.com, axboe@kernel.dk, linux-mm@kvack.org, linux-arch@vger.kernel.org, Clark Williams , Ingo Molnar Message-ID: <20080421120004.wCKrDu-GkBVG6KhNZYjbPPpK0jn-qBDKeYsRm3tnMsI@z> Linus Torvalds wrote: > Finally, I don't think that comment is correct in the first place. It's > not that simple. The thing is, even *with* the memory barrier in place, we > may have: > > CPU#1 CPU#2 > ===== ===== > > fast_gup: > - read low word > > native_set_pte_present: > - set low word to 0 > - set high word to new value > > - read high word > > - set low word to new value > > and so you read a low word that is associated with a *different* high > word! Notice? > > So trivial memory ordering is _not_ enough. > > So I think the code literally needs to be something like this > > #ifdef CONFIG_X86_PAE > > static inline pte_t native_get_pte(pte_t *ptep) > { > pte_t pte; > > retry: > pte.pte_low = ptep->pte_low; > smp_rmb(); > pte.pte_high = ptep->pte_high; > smp_rmb(); > if (unlikely(pte.pte_low != ptep->pte_low) > goto retry; > return pte; > } > > I think this is still broken. Suppose that after reading pte_high native_set_pte() is called again on another cpu, changing pte_low back to the original value (but with a different pte_high). You now have pte_low from second native_set_pte() but pte_high from the first native_set_pte(). You could use cmpxchg8b to atomically load the pte, but at the expense of taking the cacheline for exclusive access. -- error compiling committee.c: too many arguments to function