linux-kernel - Re: [PATCH] writeback: guard against jiffies wraparound on inode->dirtied

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Message-Id: <20090331172031.b2971f1e.akpm@linux-foundation.org>
Date:	Tue, 31 Mar 2009 17:20:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] writeback: guard against jiffies wraparound on
 inode->dirtied_when checks (try #2)

On Tue, 31 Mar 2009 20:03:59 -0400
Jeff Layton <jlayton@...hat.com> wrote:

> +		 * It's not sufficient to just do a time_after() check on
> +		 * dirtied_when. That assumes that dirtied_when will always
> +		 * change within a period of jiffies that encompasses half the
> +		 * machine word size (2^31 jiffies on 32-bit arch). That's not
> +		 * necessarily the case if an inode is being constantly
> +		 * redirtied. Since dirtied_when can never be in the future,
> +		 * we can assume that if it appears to be so then it is
> +		 * actually in the distant past.

so this really is a 32-bit-only thing.

I guess that isn't worth optimising for though.

otoh, given that all three comparisons are the same:

+			time_after(inode->dirtied_when, *older_than_this) &&
+			time_before_eq(inode->dirtied_when, jiffies))

(although one is inverted (i think?)), it might end up nicer if this was all done
in a little helper function?

That way we only need to comment what's going on at a single site, and
we could omit the additional test if !CONFIG_64BIT.
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