linux-kernel - Re: How is Code in do_sys_settimeofday() safe in case of SMP and Nest Kernel Path?

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Date:	Sun, 01 Oct 2006 00:37:55 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Dong Feng <middle.fengdong@...il.com>
CC:	Christoph Lameter <clameter@....com>, Andi Kleen <ak@...e.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: How is Code in do_sys_settimeofday() safe in case of SMP and
 Nest Kernel Path?

Dong Feng wrote:
> 2006/9/30, Christoph Lameter <clameter@....com>:
> 
>> On Fri, 29 Sep 2006, Dong Feng wrote:
>>

>> > If the comments mean the subsequent code is SMP-safe and can prevent
>> > nest-kernel-path, how does it achieves that?
>>
>> It relies on locking outside of do_sys_settimeofday(). Seems that this
>> indicates locking is to be performed by the arch before calling
>> do_sys_settimeofday. Looks suspicious to me. Check that this function is
>> always called with the same lock.
>>
> 
> Yes, that is the question. The whole invocation path is
> sys_settimeofday() -> do_sys_settimeofday()
> 
> I do not find a lock embracing do_sys_settimeofday().
> 
> Moreover, seems neither write operations nor read operations on sys_tz
> is protected by any locks, in sys_gettimeofday() and
> sys_settimeofday() respectively.

Did you get to the bottom of this yet? It looks like you're right,
and I suggest a seqlock might be a good option.

You should write a patch and send it to Mister Morton.

-- 
SUSE Labs, Novell Inc.
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