[PATCH] sparc32: fix broken set_pte()

[Kirill A. Shutemov]

32-bit sparc uses swap instruction to implement set_pte(). It called
using GCC inline assembler. But it misses the "memory" clobber to
indicate that pte value will be updated in memory.

As result GCC doesn't know that it cannot postpone pte pointer
dereference which occurs before set_pte() to post-set_pte() time.

It leads to real-world bugs -- [1]. In this situation we have code:

	ptent = ptep_modify_prot_start(mm, addr, pte);
	ptent = pte_modify(ptent, newprot);
	...
	ptep_modify_prot_commit(mm, addr, pte, ptent);

ptep_modify_prot_start() in sparc case is just 'pte' dereference plus
pte_clear(). pte_clear() calls broken set_pte(). GCC thinks it's valid
to dereference 'pte' again on pte_modify() and gets cleared pte.
ptep_modify_prot_commit() puts 'pteent' with pfn==0 back to page table,
which eventually leads to the crash.

[1] http://lkml.kernel.org/r/54C06B19.8060305@xxxxxxxxxxxx

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
---
 arch/sparc/include/asm/pgtable_32.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/sparc/include/asm/pgtable_32.h b/arch/sparc/include/asm/pgtable_32.h
index b2f7dc46a7d1..9912eb0b499a 100644
--- a/arch/sparc/include/asm/pgtable_32.h
+++ b/arch/sparc/include/asm/pgtable_32.h
@@ -102,7 +102,8 @@ extern unsigned long empty_zero_page;
  */
 static inline unsigned long srmmu_swap(unsigned long *addr, unsigned long value)
 {
-	__asm__ __volatile__("swap [%2], %0" : "=&r" (value) : "0" (value), "r" (addr));
+	__asm__ __volatile__("swap [%2], %0" :
+			"=&r" (value) : "0" (value), "r" (addr) : "memory");
 	return value;
 }
 
-- 
 Kirill A. Shutemov
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