From mboxrd@z Thu Jan 1 00:00:00 1970 From: akpm@linux-foundation.org Subject: + x86-avoid-constant_test_bit-misoptimization-due-to-cast-to-non-volatile.patch added to -mm tree Date: Thu, 23 Sep 2010 16:51:54 -0700 Message-ID: <201009232351.o8NNpsxB026809@imap1.linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Return-path: Received: from smtp1.linux-foundation.org ([140.211.169.13]:34001 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754838Ab0IWXxK (ORCPT ); Thu, 23 Sep 2010 19:53:10 -0400 Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: mm-commits@vger.kernel.org Cc: led@altlinux.ru, gcosta@redhat.com, hpa@zytor.com, ledest@gmail.com, mike@osdn.org.ua, mingo@elte.hu, tglx@linutronix.de, torvalds@linux-foundation.org, volodymyrgl@gmail.com 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 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 Cc: Volodymyr G. Lukiianyk Cc: Alexander Chumachenko Cc: Ingo Molnar Cc: Thomas Gleixner Cc: "H. Peter Anvin" Cc: Linus Torvalds Cc: Glauber de Oliveira Costa Signed-off-by: Andrew Morton --- 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@altlinux.ru are x86-avoid-constant_test_bit-misoptimization-due-to-cast-to-non-volatile.patch