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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 c4295fbb60) 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>
This commit is contained in:
Alexander Chumachenko 2010-04-01 15:34:52 +03:00 committed by H. Peter Anvin
parent 7329cf0201
commit c9e2fbd909
1 changed files with 1 additions and 1 deletions
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2
arch/x86/include/asm/bitops.h
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@ -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)