The patch titled x86: avoid 'constant_test_bit()' misoptimization due to cast to non-volatile has been added to the -mm tree. Its filename is x86-avoid-constant_test_bit-misoptimization-due-to-cast-to-non-volatile.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** See http://userweb.kernel.org/~akpm/stuff/added-to-mm.txt to find out what to do about this The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: x86: avoid 'constant_test_bit()' misoptimization due to cast to non-volatile From: Led <led@xxxxxxxxxxx> While debugging bit_spin_lock() hang, it was tracked down to gcc-4.4 misoptimization of constant_test_bit() 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. Other arches than asm-x86 may implement this slightly differently; 2.6.29 mitigates the misoptimization by changing the function prototype in commit c4295fbb6048 ("x86: make 'constant_test_bit()' take an unsigned bit number") but probably fixing the issue itself is better. Cc: Michael Shigorin <mike@xxxxxxxxxxx> Cc: Volodymyr G. Lukiianyk <volodymyrgl@xxxxxxxxx> Cc: Alexander Chumachenko <ledest@xxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Glauber de Oliveira Costa <gcosta@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- arch/x86/include/asm/bitops.h | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff -puN arch/x86/include/asm/bitops.h~x86-avoid-constant_test_bit-misoptimization-due-to-cast-to-non-volatile arch/x86/include/asm/bitops.h --- a/arch/x86/include/asm/bitops.h~x86-avoid-constant_test_bit-misoptimization-due-to-cast-to-non-volatile +++ a/arch/x86/include/asm/bitops.h @@ -308,8 +308,7 @@ static inline int test_and_change_bit(in 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; + return ((1UL << (nr % BITS_PER_LONG)) & addr[nr / BITS_PER_LONG]) != 0; } static inline int variable_test_bit(int nr, volatile const unsigned long *addr) _ Patches currently in -mm which might be from led@xxxxxxxxxxx are x86-avoid-constant_test_bit-misoptimization-due-to-cast-to-non-volatile.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html