Messages in this thread | | | Subject | Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last() | From | Vlastimil Babka <> | Date | Wed, 7 Dec 2016 09:48:52 +0100 |
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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?
> 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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