linux-kernel - Re: [rfc] optimise unlock

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Message-ID: <9948.1178626415@redhat.com>
Date:	Tue, 08 May 2007 13:13:35 +0100
From:	David Howells <dhowells@...hat.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	linux-arch@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [rfc] optimise unlock_page 


Nick Piggin <npiggin@...e.de> wrote:

> This patch trades a page flag for a significant improvement in the unlock_page
> fastpath. Various problems in the previous version were spotted by Hugh and
> Ben (and fixed in this one).

It looks reasonable at first glance, though it does consume yet another page
flag:-/  However, I think that's probably a worthy trade.

>  }
> -	
> +
> +static inline void unlock_page(struct page *page)
> +{
> +	VM_BUG_ON(!PageLocked(page));
> +	ClearPageLocked_Unlock(page);
> +	if (unlikely(PageWaiters(page)))
> +		__unlock_page(page);
> +}
> +

Please don't simply discard the documentation, we have little enough as it is:

> -/**
> - * unlock_page - unlock a locked page
> - * @page: the page
> - *
> - * Unlocks the page and wakes up sleepers in ___wait_on_page_locked().
> - * Also wakes sleepers in wait_on_page_writeback() because the wakeup
> - * mechananism between PageLocked pages and PageWriteback pages is shared.
> - * But that's OK - sleepers in wait_on_page_writeback() just go back to sleep.
> - *
> - * The mb is necessary to enforce ordering between the clear_bit and the read
> - * of the waitqueue (to avoid SMP races with a parallel wait_on_page_locked()).

David
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