linux-kernel - Re: [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock

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Date:	Tue, 13 May 2014 13:53:13 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	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>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 19/19] mm: filemap: Avoid unnecessary barries and
 waitqueue lookups in unlock_page fastpath

On Tue, May 13, 2014 at 10:45:50AM +0100, Mel Gorman wrote:
>  void unlock_page(struct page *page)
>  {
> +	wait_queue_head_t *wqh = clear_page_waiters(page);
> +
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
> +
> +	/*
> +	 * No additional barrier needed due to clear_bit_unlock barriering all updates
> +	 * before waking waiters
> +	 */
>  	clear_bit_unlock(PG_locked, &page->flags);
> -	smp_mb__after_clear_bit();
> -	wake_up_page(page, PG_locked);

This is wrong. The smp_mb__after_clear_bit() is still required to ensure
that the cleared bit is visible before the wakeup on all architectures.

---8<---
diff --git a/mm/filemap.c b/mm/filemap.c
index 6ac066e..028b5a1 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -819,11 +819,8 @@ void unlock_page(struct page *page)
 
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 
-	/*
-	 * No additional barrier needed due to clear_bit_unlock barriering all updates
-	 * before waking waiters
-	 */
 	clear_bit_unlock(PG_locked, &page->flags);
+	smp_mb__after_clear_bit();
 
 	/*
 	 * Wake the queue if waiters were detected. Ordinarily this wakeup
--
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