Re: [PATCH] futex: Reduce the scope of lock_page, aka lockless futex_get_key()

[Davidlohr Bueso]

On Tue, 05 Jan 2016, Bueso wrote:

> On Tue, 05 Jan 2016, Peter Zijlstra wrote:
>
> > On Tue, Jan 05, 2016 at 12:23:55PM -0800, Davidlohr Bueso wrote:
> > > +	if (unlikely(!mapping)) {
> > > +		int shmem_swizzled;
> > > +
> > > +		/*
> > > +		 * Page lock is required to identify which special case above
> > > +		 * applies. If this is really a shmem page then the page lock
> > > +		 * will prevent unexpected transitions.
> > > +		 */
> > > +		lock_page(page);
> > > +		shmem_swizzled = PageSwapCache(page);
> > > 		unlock_page(page);
> > > 		put_page(page);
> > > +		WARN_ON_ONCE(mapping);
> >
> > We've not re-loaded mapping, so how could this possibly be?
>
> Yep, this wants to be page->mapping.

Actually under compound_head() afaict, along with the rest of the page->mapping
loads.
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