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

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Date:	Wed, 27 Jun 2007 14:21:39 -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 06/26, Chris Snook wrote:
>> 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.  
> 
> I can't understand this.
> 
> Fisrt, smp_rmb() can't help in this case. It can't influence the preceeding
> LOAD if it was from cache.
> 
> Even if it could, another CPU can alter the value just after the reading
> completes, and we have the same situation.
> 
> Could you please clarify if I am wrong?
> 
> Oleg.
> 

You're right, but so is Ingo's patch.  We're not trying to enforce some notion 
of absolute time, just  make it possible for userspace to guarantee that time 
cannot be *observed* to travel backwards.  It's still the responsibility of the 
user to use proper synchronization in multithreaded apps.  Without the smp_rmb() 
it would be possible on some architectures for the results of the race you 
describe to leak across other lock-prefixed instructions used to ensure 
monotonicity in userspace.  Relativity applies to SMP timekeeping, not just 
space travelers, so if there's no way to prove a race occurred, it doesn't 
matter whether or not it occurred in some frame of reference.

	-- Chris
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