linux-kernel - Re: [PATCH v4] futex: Remove requirement for lock_page in get_futex

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Date:	Wed, 27 Jan 2016 21:10:02 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <dbueso@...e.de>
cc:	Mel Gorman <mgorman@...hsingularity.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Chris Mason <clm@...com>, Darren Hart <dvhart@...ux.intel.com>,
	dave@...olabs.net, linux-kernel@...r.kernel.org,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v4] futex: Remove requirement for lock_page in
 get_futex_key

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