linux-kernel - Re: [PATCH 17/17] mm: filemap: Avoid unnecessary barries and waitqueue lookup in unlock

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Message-ID: <20140506203006.GF1429@laptop.programming.kicks-ass.net>
Date:	Tue, 6 May 2014 22:30:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 17/17] mm: filemap: Avoid unnecessary barries and
 waitqueue lookup in unlock_page fastpath

On Thu, May 01, 2014 at 09:44:48AM +0100, Mel Gorman wrote:
> +/*
> + * If PageWaiters was found to be set at unlock time, __wake_page_waiters
> + * should be called to actually perform the wakeup of waiters.
> + */
> +static inline void __wake_page_waiters(struct page *page)
> +{
> +	ClearPageWaiters(page);

-ENOCOMMENT

barriers should always come with a comment that explain the memory
ordering and reference the pairing barrier.

Also, FWIW, there's a mass rename queued for .16 that'll make this:

  smp_mb__after_atomic();

but for now it will also still provide the old names with a __deprecated
tag on, so no real harm.

> +	smp_mb__after_clear_bit();
> +	wake_up_page(page, PG_locked);
> +}
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