linux-kernel - Re: [PATCH] kernel/sysctl_binary.c: improve the usage of return value 'result'

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Message-ID: <5201DB0D.7080200@gmail.com>
Date:	Wed, 07 Aug 2013 13:28:45 +0800
From:	Chen Gang F T <chen.gang.flying.transformer@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Chen Gang <gang.chen@...anux.com>,
	Al Viro <viro@...iv.linux.org.uk>, xi.wang@...il.com,
	nicolas.dichtel@...nd.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/sysctl_binary.c: improve the usage of return value
 'result'

On 08/07/2013 06:13 AM, Eric W. Biederman wrote:
> Andrew Morton <akpm@...ux-foundation.org> writes:
> 
>> On Tue, 06 Aug 2013 15:29:42 +0800 Chen Gang <gang.chen@...anux.com> wrote:
>>
>>> Improve the usage of return value 'result', so not only can make code
>>> clearer to readers, but also can improve the performance.
>>
>> It used to be pervasive kernel style do to
>>
>> 	ret = -ENOMEM;
>> 	foo = alloc(...);
>> 	if (!foo)
>> 		goto out;
>>
>> whereas nowadays people usually do the more straightforward
>>
>> 	foo = alloc(...);
>> 	if (!foo) {
>> 		ret = -ENOMEM;
>> 		goto out;
>> 	}
>>
>> The thinking was that the old style generated better code, but for the
>> life of me I can't remember why :(
> 
> Because doing the assignment outside of the if() goto .  Allows the
> compiler to emit the if() goto as a single branch.
> 
> While a smart compiler may perform the code motion across the branch,
> it is much easier for the compiler to branch to somewhere else perform
> the assignment and then branch out.
> 

For my opinion, for assembly code, the old style is clearer than the new
style. And commonly, the old style will be faster than new style.

Thanks.

> Eric
> 
> 
>> Your patch switches from old-style to new-style.  And it appears to
>> have increased the text size.  I did this, to switch three sites back
>> to old-style:
>>
>> --- a/kernel/sysctl_binary.c~kernel-sysctl_binaryc-improve-the-usage-of-return-value-result-fix
>> +++ a/kernel/sysctl_binary.c
>> @@ -941,17 +941,15 @@ static ssize_t bin_string(struct file *f
>>  		copied = result;
>>  		lastp = oldval + copied - 1;
>>  
>> -		if (get_user(ch, lastp)) {
>> -			result = -EFAULT;
>> +		result = -EFAULT;
>> +		if (get_user(ch, lastp))
>>  			goto out;
>> -		}
>>  
>>  		/* Trim off the trailing newline */
>>  		if (ch == '\n') {
>> -			if (put_user('\0', lastp)) {
>> -				result = -EFAULT;
>> +			result = -EFAULT;
>> +			if (put_user('\0', lastp))
>>  				goto out;
>> -			}
>>  			copied -= 1;
>>  		}
>>  	}
>> @@ -976,11 +974,10 @@ static ssize_t bin_intvec(struct file *f
>>  	char *buffer;
>>  	ssize_t result;
>>  
>> +	result = -ENOMEM;
>>  	buffer = kmalloc(BUFSZ, GFP_KERNEL);
>> -	if (!buffer) {
>> -		result = -ENOMEM;
>> +	if (!buffer)
>>  		goto out;
>> -	}
>>  
>>  	if (oldval && oldlen) {
>>  		unsigned __user *vec = oldval;
>> _
>>
>> and kernel/sysctl_binary.o's .text got six bytes smaller.
>>
>> Now, smaller text doesn't mean faster code.  But it probably means
>> larger cache footprint, which can mean slower code.
>>
>> IOW, it isn't obvious that this was an improvement.
> --
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-- 
Chen Gang
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