From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 7 May 2008 15:59:14 -0700 From: Andrew Morton Subject: Re: [PATCH 08 of 11] anon-vma-rwsem Message-Id: <20080507155914.d7790069.akpm@linux-foundation.org> In-Reply-To: <20080507224406.GI8276@duo.random> References: <6b384bb988786aa78ef0.1210170958@duo.random> <20080507212650.GA8276@duo.random> <20080507222205.GC8276@duo.random> <20080507153103.237ea5b6.akpm@linux-foundation.org> <20080507224406.GI8276@duo.random> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Andrea Arcangeli Cc: torvalds@linux-foundation.org, clameter@sgi.com, steiner@sgi.com, holt@sgi.com, npiggin@suse.de, a.p.zijlstra@chello.nl, kvm-devel@lists.sourceforge.net, kanojsarcar@yahoo.com, rdreier@cisco.com, swise@opengridcomputing.com, linux-kernel@vger.kernel.org, avi@qumranet.com, linux-mm@kvack.org, general@lists.openfabrics.org, hugh@veritas.com, rusty@rustcorp.com.au, aliguori@us.ibm.com, chrisw@redhat.com, marcelo@kvack.org, dada1@cosmosbay.com, paulmck@us.ibm.com List-ID: On Thu, 8 May 2008 00:44:06 +0200 Andrea Arcangeli wrote: > On Wed, May 07, 2008 at 03:31:03PM -0700, Andrew Morton wrote: > > Nope. We only need to take the global lock before taking *two or more* of > > the per-vma locks. > > > > I really wish I'd thought of that. > > I don't see how you can avoid taking the system-wide-global lock > before every single anon_vma->lock/i_mmap_lock out there without > mm_lock. > > Please note, we can't allow a thread to be in the middle of > zap_page_range while mmu_notifier_register runs. > > vmtruncate takes 1 single lock, the i_mmap_lock of the inode. Not more > than one lock and we've to still take the global-system-wide lock > _before_ this single i_mmap_lock and no other lock at all. > > Please elaborate, thanks! umm... CPU0: CPU1: spin_lock(a->lock); spin_lock(b->lock); spin_lock(b->lock); spin_lock(a->lock); bad. CPU0: CPU1: spin_lock(global_lock) spin_lock(global_lock); spin_lock(a->lock); spin_lock(b->lock); spin_lock(b->lock); spin_lock(a->lock); Is OK. CPU0: CPU1: spin_lock(global_lock) spin_lock(a->lock); spin_lock(b->lock); spin_lock(b->lock); spin_unlock(b->lock); spin_lock(a->lock); spin_unlock(a->lock); also OK. As long as all code paths which can take two-or-more locks are all covered by the global lock there is no deadlock scenario. If a thread takes just a single instance of one of these locks without taking the global_lock then there is also no deadlock. Now, if we need to take both anon_vma->lock AND i_mmap_lock in the newly added mm_lock() thing and we also take both those locks at the same time in regular code, we're probably screwed. -- 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