Messages in this thread | | | Date | Wed, 7 Dec 2016 09:43:05 +0100 | From | Michal Hocko <> | Subject | Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last() |
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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@huawei.com> > Suggested-by: Christian Borntraeger <borntraeger@de.ibm.com> > --- > 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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