linux-kernel - Re: Apparent backward time travel in timestamps on file creation

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Message-ID: <26743.1490912537@warthog.procyon.org.uk>
Date:   Thu, 30 Mar 2017 23:22:17 +0100
From:   David Howells <dhowells@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     dhowells@...hat.com, Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: Apparent backward time travel in timestamps on file creation

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> The "is it in sync with gettimeofday()" is interesting too, even if
> the answer is that you don't expect it to be _perfectly_ in sync. A
> test that just reports maximum slop might be an interesting test, and
> could show real problems (maybe bad network time synchronization, but
> maybe actual bugs in our internal xtime handling even for local
> filesystems!).

I wonder if multi-cpu systems might show interesting differences between CPUs
too.  I would hope not since xtime is based on a global variable.

> And then if your tool starts reporting times that are off by seconds
> or minutes, people might say "Hey, that's not right.." and find
> something.

More likely never see it as the output is hidden away by xfstests.  Probably
xfstests needs to gain some way of lending prominence to information of this
type.

David