linux-kernel - Re: [PATCH v3] bitops: Fix shift overflow in GENMASK macros

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Date:	Thu, 6 Nov 2014 10:56:57 +0100
From:	Maxime Coquelin <maxime.coquelin@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	<gong.chen@...ux.intel.com>, Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, <tytso@....edu>,
	<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
	<kernel@...inux.com>, <eric.paire@...com>
Subject: Re: [PATCH v3] bitops: Fix shift overflow in GENMASK macros


On 11/05/2014 05:44 PM, Peter Zijlstra wrote:
> On Wed, Nov 05, 2014 at 04:47:14PM +0100, Maxime Coquelin wrote:
>> On 11/05/2014 12:10 PM, Rasmus Villemoes wrote:
>>> On Tue, Nov 04 2014, Maxime COQUELIN <maxime.coquelin@...com> wrote:
>>>
>>>> -#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 - ((h) - (l) + 1))) << (l))
>>>> +
>>>> +#define GENMASK_ULL(h, l) \
>>>> +	((~0ULL >> (BITS_PER_LONG_LONG - ((h) - (l) + 1))) << (l))
>>> Slightly bikeshedding here, but may I suggest spelling it
>>>
>>>    (((~0UL) << (l)) & (~0UL >> (BITS_PER_LONG - 1 - (h))))
>>>
>>> ? This will ensure the arguments are expanded once each, and will, IMHO,
>>> DTRT if/when l > h (namely, yield 0).
>> That's a good point.
>>
>> Peter, what is your view?
> I'm fine with that, feel free to post another version and I'll replace
> v3.

Thanks Peter, v4 just sent.

Regards,
Maxime
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