On Tue 06-12-16 09:53:14, Xishi Qiu wrote:
> A compiler could re-read "old_flags" from the memory location after reading
> and calculation "flags" and passes a newer value into the cmpxchg making
> the comparison succeed while it should actually fail.
>
> Signed-off-by: Xishi Qiu <qiuxishi@xxxxxxxxxx>
> Suggested-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> ---
> mm/mmzone.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/mmzone.c b/mm/mmzone.c
> index 5652be8..e0b698e 100644
> --- a/mm/mmzone.c
> +++ b/mm/mmzone.c
> @@ -102,7 +102,7 @@ int page_cpupid_xchg_last(struct page *page, int cpupid)
> int last_cpupid;
>
> do {
> - old_flags = flags = page->flags;
> + old_flags = flags = READ_ONCE(page->flags);
> last_cpupid = page_cpupid_last(page);
what prevents compiler from doing?
old_flags = READ_ONCE(page->flags);
flags = READ_ONCE(page->flags);
Or this doesn't matter?
>
> flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
> --
> 1.8.3.1
>
--
Michal Hocko
SUSE Labs
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