Re: [PATCH] writeback: guard against jiffies wraparound on inode->dirtied_when checks (try #2)

Andrew Morton
2009-04-01T00:23:35Z

On Tue, 31 Mar 2009 20:03:59 -0400
Jeff Layton <jlayton@redhat.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.