linux-kernel - Re: Proposal: Use hi-res clock for file timestamps

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Date:	Tue, 17 Aug 2010 20:29:20 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"Patrick J. LoPresti" <lopresti@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Proposal: Use hi-res clock for file timestamps

> OK, so that leaves us with the race, even on newer filesystems:
> 
> 	1. File is modified, mtime updated
> 	2. Client fetches mtime to revalidate cache
> 	3. File is modified again, mtime updated
> 	4. Client fetches new mtime to revalidate cache

You'll always have a race window with time, the only way around
that would be a version number.
> 	- Tell everyone to use NFSv4 (and make sure we have
> 	  changeattr/i_version working correctly).
> 	- Use a finer-grained time source.  (I believe you when you say
> 	  the TSC is too slow, but maybe we should run some tests to
> 	  make sure.)

It depends on the CPU too.

> 	- Increment mtime by a nanosecond when necessary.  

You cannot be more precise than the backing file system: this causes
non monotonity when the inodes are flushed (has happened in the past)

-Andi
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