linux-kernel - Re: [PATCH v3] signals: Avoid unnecessary taking of sighand->siglock

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Date:   Tue, 27 Sep 2016 14:21:56 -0400
From:   Waiman Long <waiman.long@....com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stas Sergeev <stsp@...t.ru>, <linux-kernel@...r.kernel.org>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v3] signals: Avoid unnecessary taking of sighand->siglock

On 09/27/2016 12:17 PM, Oleg Nesterov wrote:
> On 09/27, Waiman Long wrote:
>> +static inline int sigequalsets(const sigset_t *set1, const sigset_t *set2)
>> +{
>> +	switch (_NSIG_WORDS) {
>> +	case 4:
>> +		return	(set1->sig[3] == set2->sig[3])&&
>> +			(set1->sig[2] == set2->sig[2])&&
>> +			(set1->sig[1] == set2->sig[1])&&
>> +			(set1->sig[0] == set2->sig[0]);
>> +	case 2:
>> +		return	(set1->sig[1] == set2->sig[1])&&
>> +			(set1->sig[0] == set2->sig[0]);
>> +	case 1:
>> +		return	set1->sig[0] == set2->sig[0];
>> +	}
>> +	return 0;
>> +}
>> +
> OK, this memcmp-by-hand matches other sig* helpers. Well, perhaps
>
> 	default:
> 		BUILD_BUG();
>
> makes sense too, but I won't insist.

We already have a BUILD_BUG() call in sigemptyset(). I don't think we 
need more than one in any given source file. The memcmp() call will be 
more efficient for long byte stream. For short one like sigset_t, direct 
comparison is likely to be faster.

> Acked-by: Oleg Nesterov <oleg@...hat.com> 

Thanks for the review.

Cheers,
Longman