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