Re: [PATCH v2] fix __set_page_dirty_no_writeback() return value

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 11 Nov 2010 11:05:54 +0800 Bob Liu <lliubbo@gmail.com> wrote:

> __set_page_dirty_no_writeback() should return true if it actually transitioned
> the page from a clean to dirty state although it seems nobody used its return
> value now.
> 
> Change from v1:
> 	* preserving cacheline optimisation as Andrew pointed out
> 
> Signed-off-by: Bob Liu <lliubbo@gmail.com>
> ---
>  mm/page-writeback.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index bf85062..ac7018a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1157,8 +1157,10 @@ EXPORT_SYMBOL(write_one_page);
>   */
>  int __set_page_dirty_no_writeback(struct page *page)
>  {
> -	if (!PageDirty(page))
> +	if (!PageDirty(page)) {
>  		SetPageDirty(page);
> +		return 1;
> +	}
>  	return 0;
>  }

But that has a race.  If someone else sets PG_Dirty between the test
and the set, this function will incorrectly return 1.

Which is why it should use test_and_set if we're going to do this.

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