linux-kernel - Re: [PATCH] futex: Reduce the scope of lock_page, aka lockless futex_get

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Date:	Tue, 5 Jan 2016 13:11:40 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mel Gorman <mgorman@...hsingularity.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Chris Mason <clm@...com>, Darren Hart <dvhart@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] futex: Reduce the scope of lock_page, aka lockless
 futex_get_key()

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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