linux-kernel - Re: [PATCH 08 of 11] anon-vma-rwsem

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Message-Id: <20080507153103.237ea5b6.akpm@linux-foundation.org>
Date:	Wed, 7 May 2008 15:31:03 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andrea Arcangeli <andrea@...ranet.com>
Cc:	torvalds@...ux-foundation.org, clameter@....com, steiner@....com,
	holt@....com, npiggin@...e.de, a.p.zijlstra@...llo.nl,
	kvm-devel@...ts.sourceforge.net, kanojsarcar@...oo.com,
	rdreier@...co.com, swise@...ngridcomputing.com,
	linux-kernel@...r.kernel.org, avi@...ranet.com, linux-mm@...ck.org,
	general@...ts.openfabrics.org, hugh@...itas.com,
	rusty@...tcorp.com.au, aliguori@...ibm.com, chrisw@...hat.com,
	marcelo@...ck.org, dada1@...mosbay.com, paulmck@...ibm.com
Subject: Re: [PATCH 08 of 11] anon-vma-rwsem

On Thu, 8 May 2008 00:22:05 +0200
Andrea Arcangeli <andrea@...ranet.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.
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