Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last()

[Michal Hocko <mhocko@kernel.org>]

On Wed 07-12-16 10:40:47, Christian Borntraeger wrote:
> On 12/07/2016 10:29 AM, Vlastimil Babka wrote:
> > On 12/07/2016 09:58 AM, Michal Hocko wrote:
> >> On Wed 07-12-16 09:48:52, Vlastimil Babka wrote:
> >>> On 12/07/2016 09:43 AM, Michal Hocko wrote:
> >>>> 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);
> >>>
> >>> AFAIK, READ_ONCE tells the compiler that page->flags is volatile. It
> >>> can't read from volatile location more times than being told?
> >>
> >> But those are two different variables which we assign to so what
> >> prevents the compiler from applying READ_ONCE on each of them
> >> separately?
> > 
> > I would naively expect that it's assigned to flags first, and then from
> > flags to old_flags. But I don't know exactly the C standard evaluation
> > rules that apply here.
> > 
> >> Anyway, this could be addressed easily by
> > 
> > Yes, that way there should be no doubt.
> 
> That change would make it clearer, but the code is correct anyway,
> as assignments in C are done from right to left, so 
> old_flags = flags = READ_ONCE(page->flags);
> 
> is equivalent to 
> 
> flags = READ_ONCE(page->flags);
> old_flags = flags;

OK, I guess you are right. For some reason I thought that the compiler
is free to bypass flags and split an assignment
a = b = c; into b = c; a = c
which would still follow from right to left rule. I guess I am over
speculating here though, so sorry for the noise.
-- 
Michal Hocko
SUSE Labs

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