linux-kernel - Re: [PATCH] sys_time-speedup-small-cleanup

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Date:	Tue, 26 Jun 2007 12:21:48 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Oleg Nesterov <oleg@...sign.ru>
CC:	Ingo Molnar <mingo@...e.hu>, John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sys_time-speedup-small-cleanup

Oleg Nesterov wrote:
> on top of sys_time-speedup.patch
> 
> Ingo Molnar wrote:
>>  asmlinkage long sys_time(time_t __user * tloc)
>>  {
>> -	time_t i;
>> -	struct timeval tv;
>> +	/*
>> +	 * We read xtime.tv_sec atomically - it's updated
>> +	 * atomically by update_wall_time(), so no need to
>> +	 * even read-lock the xtime seqlock:
>> +	 */
>> +	time_t i = xtime.tv_sec;
>>  
>> -	do_gettimeofday(&tv);
>> -	i = tv.tv_sec;
>> +	smp_rmb(); /* sys_time() results are coherent */
> 
> Why do we need this barrier? My guess it is needed to prevent
> the reading of xtime.tv_sec twice, yes? In that case a simple
> barrier() should be enough.

Without the smp_rmb, you can potentially have a situation where one CPU is still 
reading an old value from cache while another has the new value.  It's generally 
a rather small race window on most architectures, but very bad things can happen 
if time ever goes backwards, so it's worth the overhead of maintaining coherence 
on smp.

	-- Chris

> Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
> 
> --- t/kernel/time.c~	2007-06-26 16:28:59.000000000 +0400
> +++ t/kernel/time.c	2007-06-26 16:32:09.000000000 +0400
> @@ -64,7 +64,7 @@ asmlinkage long sys_time(time_t __user *
>  	 */
>  	time_t i = xtime.tv_sec;
>  
> -	smp_rmb(); /* sys_time() results are coherent */
> +	barrier(); /* sys_time() results are coherent */
>  
>  	if (tloc) {
>  		if (put_user(i, tloc))
> 
> -
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