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


    >
    >> diff --git a/mm/mmzone.c b/mm/mmzone.c
    >> index 5652be858e5e..b4e093dd24c1 100644
    >> --- a/mm/mmzone.c
    >> +++ b/mm/mmzone.c
    >> @@ -102,10 +102,10 @@ int page_cpupid_xchg_last(struct page *page, int cpupid)
    >> int last_cpupid;
    >>
    >> do {
    >> - old_flags = flags = page->flags;
    >> + old_flags = READ_ONCE(page->flags);
    >> last_cpupid = page_cpupid_last(page);
    >>
    >> - flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
    >> + flags = old_flags & ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
    >> flags |= (cpupid & LAST_CPUPID_MASK) << LAST_CPUPID_PGSHIFT;
    >> } while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags));
    >>
    >>
    >>>> Or this doesn't matter?
    >>>
    >>> I think it would matter.
    >>>
    >>>>>
    >>>>> flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
    >>>>> --
    >>>>> 1.8.3.1
    >>>>>
    >>>>
    >>
    >

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