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

[Michal Hocko]

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@xxxxxxxxxx>
> >> Suggested-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> >> ---
> >>  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? Anyway, this could be addressed easily by 
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
> >>
> > 

-- 
Michal Hocko
SUSE Labs