From: Alexander Chumachenko Date: Thu, 1 Apr 2010 12:34:52 +0000 (+0300) Subject: x86: Avoid 'constant_test_bit()' misoptimization due to cast to non-volatile X-Git-Tag: v2.6.36-rc6~7^2 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=c9e2fbd909c20b165b2b9ffb59f8b674cf0a55b0;p=profile%2Fcommon%2Fkernel-common.git x86: Avoid 'constant_test_bit()' misoptimization due to cast to non-volatile 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 c4295fbb6048d85f0b41c5ced5cbf63f6811c46c) but probably fixing the issue itself is better. Signed-off-by: Alexander Chumachenko Signed-off-by: Michael Shigorin Acked-by: Linus Torvalds Signed-off-by: H. Peter Anvin --- diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 545776e..bafd80d 100644 --- 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)