On Thu, Jun 08, 2017 at 11:38:21AM +0200, Vlastimil Babka wrote: > On 06/06/2017 07:58 PM, Will Deacon wrote: > > page_ref_freeze and page_ref_unfreeze are designed to be used as a pair, > > wrapping a critical section where struct pages can be modified without > > having to worry about consistency for a concurrent fast-GUP. > > > > Whilst page_ref_freeze has full barrier semantics due to its use of > > atomic_cmpxchg, page_ref_unfreeze is implemented using atomic_set, which > > doesn't provide any barrier semantics and allows the operation to be > > reordered with respect to page modifications in the critical section. > > > > This patch ensures that page_ref_unfreeze is ordered after any critical > > section updates, by invoking smp_mb__before_atomic() prior to the > > atomic_set. > > > > Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> > > Acked-by: Steve Capper <steve.capper@xxxxxxx> > > Signed-off-by: Will Deacon <will.deacon@xxxxxxx> > > Undecided if it's really needed. This is IMHO not the classical case > from Documentation/core-api/atomic_ops.rst where we have to make > modifications visible before we let others see them? Here the one who is > freezing is doing it so others can't get their page pin and interfere > with the freezer's work. Hm.. I'm not sure I'm getting what you are talking about. What would guarantee others to see changes to page before seeing page unfreezed? > > include/linux/page_ref.h | 1 + > > 1 file changed, 1 insertion(+) > > > > diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h > > index 610e13271918..74d32d7905cb 100644 > > --- a/include/linux/page_ref.h > > +++ b/include/linux/page_ref.h > > @@ -174,6 +174,7 @@ static inline void page_ref_unfreeze(struct page *page, int count) > > VM_BUG_ON_PAGE(page_count(page) != 0, page); > > VM_BUG_ON(count == 0); > > > > + smp_mb__before_atomic(); > > atomic_set(&page->_refcount, count); I *think* it should be smp_mb(), not __before_atomic(). atomic_set() is not really atomic. For instance on x86 it's plain WRITE_ONCE() which CPU would happily reorder. > > if (page_ref_tracepoint_active(__tracepoint_page_ref_unfreeze)) > > __page_ref_unfreeze(page, count); > > > -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
