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Message-ID: <4682AAB3.6050704@redhat.com>
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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