From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: [ofa-general] Re: [PATCH 08 of 11] anon-vma-rwsem
Date: Wed, 7 May 2008 15:31:03 -0700
Message-ID: <20080507153103.237ea5b6.akpm@linux-foundation.org>
References: <6b384bb988786aa78ef0.1210170958@duo.random>
	<alpine.LFD.1.10.0805071349200.3024@woody.linux-foundation.org>
	<20080507212650.GA8276@duo.random>
	<alpine.LFD.1.10.0805071429170.3024@woody.linux-foundation.org>
	<20080507222205.GC8276@duo.random>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Cc: npiggin@suse.de, chrisw@redhat.com, rusty@rustcorp.com.au,
	a.p.zijlstra@chello.nl, kvm-devel@lists.sourceforge.net,
	kanojsarcar@yahoo.com, rdreier@cisco.com, steiner@sgi.com,
	linux-kernel@vger.kernel.org, avi@qumranet.com,
	aliguori@us.ibm.com, paulmck@us.ibm.com, linux-mm@kvack.org,
	holt@sgi.com, general@lists.openfabrics.org, hugh@veritas.com,
	marcelo@kvack.org, dada1@cosmosbay.com,
	torvalds@linux-foundation.org, clameter@sgi.com
To: Andrea Arcangeli <andrea@qumranet.com>
Return-path: <general-bounces@lists.openfabrics.org>
In-Reply-To: <20080507222205.GC8276@duo.random>
List-Unsubscribe: <http://lists.openfabrics.org/cgi-bin/mailman/listinfo/general>,
	<mailto:general-request@lists.openfabrics.org?subject=unsubscribe>
List-Archive: <http://lists.openfabrics.org/pipermail/general>
List-Post: <mailto:general@lists.openfabrics.org>
List-Help: <mailto:general-request@lists.openfabrics.org?subject=help>
List-Subscribe: <http://lists.openfabrics.org/cgi-bin/mailman/listinfo/general>,
	<mailto:general-request@lists.openfabrics.org?subject=subscribe>
Sender: general-bounces@lists.openfabrics.org
Errors-To: general-bounces@lists.openfabrics.org
List-Id: kvm.vger.kernel.org

On Thu, 8 May 2008 00:22:05 +0200
Andrea Arcangeli <andrea@qumranet.com> wrote:

> > No, the simple solution is to just make up a whole new upper-level lock, 
> > and get that lock *first*. You can then take all the multiple locks at a 
> > lower level in any order you damn well please. 
> 
> Unfortunately the lock you're talking about would be:
> 
> static spinlock_t global_lock = ...
> 
> There's no way to make it more granular.
> 
> So every time before taking any ->i_mmap_lock _and_ any anon_vma->lock
> we'd need to take that extremely wide spinlock first (and even worse,
> later it would become a rwsem when XPMEM is selected making the VM
> even slower than it already becomes when XPMEM support is selected at
> compile time).

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.