linux-kernel - Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files

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Message-ID: <alpine.LFD.1.00.0801181325510.2957@woody.linux-foundation.org>
Date:	Fri, 18 Jan 2008 13:27:07 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Anton Salikhmetov <salikhmetov@...il.com>
cc:	Miklos Szeredi <miklos@...redi.hu>, peterz@...radead.org,
	linux-mm@...ck.org, jakob@...hought.net,
	linux-kernel@...r.kernel.org, valdis.kletnieks@...edu,
	riel@...hat.com, ksm@...dk, staubach@...hat.com,
	jesper.juhl@...il.com, akpm@...ux-foundation.org,
	protasnb@...il.com, r.e.wolff@...wizard.nl,
	hidave.darkstar@...il.com, hch@...radead.org
Subject: Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped
 files



On Sat, 19 Jan 2008, Anton Salikhmetov wrote:
> 
> Before using pte_wrprotect() the vma_wrprotect() routine uses the
> pte_offset_map_lock() macro to get the PTE and to acquire the ptl
> spinlock. Why did you say that this code was not SMP-safe? It should
> be atomic, I think.

It's atomic WITH RESPECT TO OTHER PEOPLE WHO GET THE LOCK.

Guess how much another x86 CPU cares when it sets the accessed bit in 
hardware?

> The POSIX standard requires the ctime and mtime stamps to be updated
> not later than at the second call to msync() with the MS_ASYNC flag.

.. and that is no excuse for bad code.

			Linus
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