Messages in this thread | | | Subject | Re: [RFC PATCH] mm: use ACCESS_ONCE in page_cpupid_xchg_last() | From | Christian Borntraeger <> | Date | Mon, 5 Dec 2016 09:31:18 +0100 |
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On 12/05/2016 09:23 AM, Xishi Qiu wrote: > By reading the code, I find the following code maybe optimized by > compiler, maybe page->flags and old_flags use the same register, > so use ACCESS_ONCE in page_cpupid_xchg_last() to fix the problem.
please use READ_ONCE instead of ACCESS_ONCE for future patches.
> > Signed-off-by: Xishi Qiu <qiuxishi@huawei.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 = ACCESS_ONCE(page->flags); > last_cpupid = page_cpupid_last(page); > > flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
I dont thing that this is actually a problem. The code below does
} while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags))
and the cmpxchg should be an atomic op that should already take care of everything (page->flags is passed as a pointer).
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