From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 7 Mar 2008 12:10:54 -0800 (PST) From: Christoph Lameter Subject: Re: [PATCH] 3/4 combine RCU with seqlock to allow mmu notifier methods to sleep (#v9 was 1/4) In-Reply-To: <20080307175019.GK24114@v2.random> Message-ID: References: <20080303220502.GA5301@v2.random> <47CC9B57.5050402@qumranet.com> <20080304133020.GC5301@v2.random> <20080304222030.GB8951@v2.random> <20080307151722.GD24114@v2.random> <20080307152328.GE24114@v2.random> <1204908762.8514.114.camel@twins> <20080307175019.GK24114@v2.random> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Andrea Arcangeli Cc: Peter Zijlstra , Jack Steiner , Nick Piggin , akpm@linux-foundation.org, Robin Holt , Avi Kivity , kvm-devel@lists.sourceforge.net, general@lists.openfabrics.org, Steve Wise , Roland Dreier , Kanoj Sarcar , linux-kernel@vger.kernel.org, linux-mm@kvack.org, daniel.blueman@quadrics.com List-ID: On Fri, 7 Mar 2008, Andrea Arcangeli wrote: > In the meantime I've also been thinking that we could need the > write_seqlock in mmu_notifier_register, to know when to restart the > loop if somebody does a mmu_notifier_register; > synchronize_rcu(). Otherwise there's no way to be sure the mmu > notifier will start firing immediately after synchronize_rcu. I'm > unsure if it's acceptable that in-progress mmu notifier invocations, > don't need to notice the fact that somebody did mmu_notifier_register; > synchronize_rcu. If they don't need to notice, then we can just drop > unregister and all rcu_read_lock()s instead of adding write_seqlock to > the register operation. This is all getting into some very complicated issues..... > Overall my effort is to try to avoid expand the list walk with > explicit memory barriers like in EMM while trying to be equally > efficient. The smp_rmb is such a big problem? You have seqlock, rcu etc all in there as well. I doubt that this is more efficient. > Another issue is that the _begin/_end logic doesn't provide any > guarantee that the _begin will start firing before _end, if a kernel > module is loaded while another cpu is already running inside some > munmap operation etc.. The KVM usage of mmu notifier has no problem > with that detail, but KVM doesn't use _begin at all, I wonder if > others would have problems. This is a kind of a separate problem, but > quite related to the question if the notifiers must be guaranteed to > start firing immediately after mmu_notifier_unregister;synchronize_rcu > or not, that's why I mentioned it here. Ahh. Yes that is an interesting issue. If a device driver cannot handle this then _begin must prohibit module loading. That means not allowing stop_machine_run I guess which should not be that difficult. -- 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