Re: [PATCH v4] futex: Remove requirement for lock_page in get_futex_key

[Thomas Gleixner]

On Sat, 23 Jan 2016, 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(READ_ONCE(page->mapping));

You just did put_page(page). So dereferencing page is a nono. This needs to be
flipped around.

Thanks,

        tglx