linux-kernel - Re: [PATCH v5] futex: Remove requirement for lock_page in get_futex

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Date:	Thu, 4 Feb 2016 10:06:11 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
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>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dbueso@...e.de>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v5] futex: Remove requirement for lock_page in
 get_futex_key

On Wed, 3 Feb 2016, Davidlohr Bueso wrote:
> +		 * We are not calling into get_futex_key_refs() in file-backed
> +		 * cases, therefore a successful atomic_inc return below will
> +		 * guarantee that get_futex_key() will continue to imply MB (B).

Can you please make that "MB (B)" part a bit more outstanding. I really had to
search for it.

> +		 */
> +		if (WARN_ON_ONCE(!atomic_inc_not_zero(&inode->i_count))) {
> +			rcu_read_unlock();
> +			put_page(page);
> +
> +			goto again;
> +		}

Don't we need 

		smp_mb__after_atomic();

here to make it a full barrier on all architectures?

Thanks,

	tglx