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

…olatile

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>

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)