Re: [Patch] memory unplug v3 [2/4] migration by kernel

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 22 May 2007 18:56:56 -0700 (PDT)
Christoph Lameter <clameter@sgi.com> wrote:

> On Wed, 23 May 2007, KAMEZAWA Hiroyuki wrote:
> 
> > > > +#ifdef CONFIG_MIGRATION_BY_KERNEL
> > > > +struct anon_vma *anon_vma_hold(struct page *page) {
> > > > +	struct anon_vma *anon_vma;
> > > > +	anon_vma = page_lock_anon_vma(page);
> > > > +	if (!anon_vma)
> > > > +		return NULL;
> > > > +	atomic_set(&anon_vma->ref, 1);
> > > 
> > > Why use an atomic value if it is set and cleared within a spinlock?
> > 
> > anon_vma_free(), which see this value, doesn't take any lock and use atomic ops.
> > I used atomic ops to handle atomic_t.
> 
> anon_vma_free() only reads the value. Thus no race. You do not need an 
> atomic_t. atomic_t is only necessary if a variable needs to be changed 
> atomically. Reading a word from memory is atomic regardless.
> 
thank you for pointing out. I understand.

-Kame

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