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

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Message-ID: <Pine.LNX.4.64.0609290903550.23840@schroedinger.engr.sgi.com>
Date:	Fri, 29 Sep 2006 09:05:57 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Dong Feng <middle.fengdong@...il.com>
cc:	Andi Kleen <ak@...e.de>, Nick Piggin <nickpiggin@...oo.com.au>,
	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?

On Fri, 29 Sep 2006, Dong Feng wrote:

> For my understanding, an assignment between structs should be a
> bit-wise copy. Such operation is not atomic, so it can not be supposed

Byte or Machine word yes.

> SMP-safe. And the subsequent test-and-assign operation on firsttime is
> not atomic, either.

No its not atomic on its own. Correct.

> 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.
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