x86: Avoid 'constant_test_bit()' misoptimization due to cast to non-volatile

Ledest · H. Peter Anvin · commit c9e2fbd909c2 · 2010-09-26T22:43:07.000-07:00
While debugging bit_spin_lock() hang, it was tracked down to gcc-4.4 misoptimization of non-inlined constant_test_bit() due to non-volatile addr when 'const volatile unsigned long *addr' cast to 'unsigned long *' with subsequent unconditional jump to pause (and not to the test) leading to hang. Compiling with gcc-4.3 or disabling CONFIG_OPTIMIZE_INLINING yields inlined constant_test_bit() and correct jump, thus working around the kernel bug. Other arches than asm-x86 may implement this slightly differently; 2.6.29 mitigates the misoptimization by changing the function prototype (commit c4295fb) but probably fixing the issue itself is better. Signed-off-by: Alexander Chumachenko <ledest@gmail.com> Signed-off-by: Michael Shigorin <mike@osdn.org.ua> Acked-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
@@ -309,7 +309,7 @@ static inline int test_and_change_bit(int nr, volatile unsigned long *addr)
 static __always_inline int constant_test_bit(unsigned int nr, const volatile unsigned long *addr)
 {
 	return ((1UL << (nr % BITS_PER_LONG)) &
-		(((unsigned long *)addr)[nr / BITS_PER_LONG])) != 0;
+		(addr[nr / BITS_PER_LONG])) != 0;
 }
 
 static inline int variable_test_bit(int nr, volatile const unsigned long *addr)

Original file line number	Diff line number	Diff line change
`@@ -309,7 +309,7 @@ static inline int test_and_change_bit(int nr, volatile unsigned long *addr)`
`309`	`309`	`static __always_inline int constant_test_bit(unsigned int nr, const volatile unsigned long *addr)`
`310`	`310`	`{`
`311`	`311`	`return ((1UL << (nr % BITS_PER_LONG)) &`
`312`		`- (((unsigned long *)addr)[nr / BITS_PER_LONG])) != 0;`
	`312`	`+ (addr[nr / BITS_PER_LONG])) != 0;`
`313`	`313`	`}`
`314`	`314`
`315`	`315`	`static inline int variable_test_bit(int nr, volatile const unsigned long *addr)`