Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752243AbaKDIlF (ORCPT ); Tue, 4 Nov 2014 03:41:05 -0500 Received: from mx08-00178001.pphosted.com ([91.207.212.93]:54992 "EHLO mx08-00178001.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750901AbaKDIlD (ORCPT ); Tue, 4 Nov 2014 03:41:03 -0500 Message-ID: <545890F8.60909@st.com> Date: Tue, 4 Nov 2014 09:40:24 +0100 From: Maxime Coquelin User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 MIME-Version: 1.0 To: Peter Zijlstra Cc: , Ingo Molnar , "Paul E. McKenney" , , , , , Subject: Re: [PATCH] bitops: Fix shift overflow in GENMASK macros References: <1415036398-26984-1-git-send-email-maxime.coquelin@st.com> <20141104082901.GF10501@worktop.programming.kicks-ass.net> In-Reply-To: <20141104082901.GF10501@worktop.programming.kicks-ass.net> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.201.23.80] X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:5.12.52,1.0.28,0.0.0000 definitions=2014-11-04_04:2014-11-03,2014-11-04,1970-01-01 signatures=0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Peter, On 11/04/2014 09:29 AM, Peter Zijlstra wrote: > On Mon, Nov 03, 2014 at 06:39:58PM +0100, Maxime COQUELIN wrote: >> On some 32 bits architectures, including x86, GENMASK(31, 0) returns 0 >> instead of the expected ~0UL. >> >> This is the same on some 64 bits architectures with GENMASK_ULL(63, 0). >> >> This is due to an overflow in the shift operand, 1 << 32 for GENMASK, >> 1 << 64 for GENMASK_ULL. >> >> Fixes: 10ef6b0dffe404bcc54e94cb2ca1a5b18445a66b >> Cc: #v3.13+ >> Reported-by: Eric Paire >> Signed-off-by: Maxime Coquelin >> --- >> include/linux/bitops.h | 8 ++++++-- >> 1 file changed, 6 insertions(+), 2 deletions(-) >> >> diff --git a/include/linux/bitops.h b/include/linux/bitops.h >> index be5fd38..81f9725 100644 >> --- a/include/linux/bitops.h >> +++ b/include/linux/bitops.h >> @@ -18,8 +18,12 @@ >> * position @h. For example >> * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000. >> */ >> -#define GENMASK(h, l) (((U32_C(1) << ((h) - (l) + 1)) - 1) << (l)) >> -#define GENMASK_ULL(h, l) (((U64_C(1) << ((h) - (l) + 1)) - 1) << (l)) >> +#define GENMASK(h, l) \ >> + ((~0UL >> ((BITS_PER_LONG - 1) - (h))) & ~((1UL << (l)) - 1)) >> + >> +#define GENMASK_ULL(h, l) \ >> + ((~0ULL >> ((BITS_PER_LONG_LONG - 1) - (h))) & ~((1ULL << (l)) - 1)) >> + > I was not expecting the mask there, but instead something like: > > ((~0UL >> (BITS_PER_LONG - (h-l+1))) << l) > > which shifts the bits to the desired length and then back to the desired > place. Would that not be more readable? > Yes, this is indeed more readable. I will send a v2 with your implementation. Thanks, Maxime -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/