Fix threaded user page write memory ordering

Make sure we order the writes to a newly created page
with the page table update that potentially exposes the
page to another CPU.

This is a no-op on any architecture where getting the
page table spinlock will already do the ordering (notably
x86), but other architectures can care.

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 232d8fdb557c..7153aef34d5c 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -40,6 +40,8 @@ static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
 	void *addr = kmap_atomic(page, KM_USER0);
 	clear_user_page(addr, vaddr, page);
 	kunmap_atomic(addr, KM_USER0);
+	/* Make sure this page is cleared on other CPU's too before using it */
+	smp_wmb();
 }
 
 static inline void clear_highpage(struct page *page)
@@ -73,6 +75,8 @@ static inline void copy_user_highpage(struct page *to, struct page *from, unsign
 	copy_user_page(vto, vfrom, vaddr, to);
 	kunmap_atomic(vfrom, KM_USER0);
 	kunmap_atomic(vto, KM_USER1);
+	/* Make sure this page is cleared on other CPU's too before using it */
+	smp_wmb();
 }
 
 static inline void copy_highpage(struct page *to, struct page *from)