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Date: Thu, 19 Jul 2007 20:41:02 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: David Howells <dhowells@redhat.com>, torvalds@linux-foundation.org,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] AFS: Fix file locking
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On Wed, 18 Jul 2007 15:56:53 +1000 Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> Andrew Morton wrote:
> > On Tue, 17 Jul 2007 13:47:32 +0100
> > David Howells <dhowells@redhat.com> wrote:
> > 
> > 
> >>+	if (type == AFS_LOCK_READ &&
> >>+	    vnode->flags & (1 << AFS_VNODE_READLOCKED)) {
> > 
> > 
> > Here we use
> > 
> > 	vnode->flags & (1 << foo)
> > 
> > 
> >>+		set_bit(AFS_VNODE_LOCKING, &vnode->flags);
> > 
> > 
> > and elsewhere we use set_bit(foo, &vnode->flags) and clear_bit()
> > 
> > This is a bit strange.  Does the open-coded bit-test have any performance
> > benefit on any architecture?  Not on x86 at least, afaik.
> 
> It uses locked operations on x86, but you can use __set_bit instead
> (which should always be at least as efficient as the C version).

I said "bit-test".  ie: test_bit().  That doesn't use a locked operation.