Re: [PATCH RFC] vfs: make fstatat retry on ESTALE errors fromgetattr call

From: Jeff Layton
Date: Tue Apr 17 2012 - 09:31:59 EST

On Tue, 17 Apr 2012 15:12:20 +0200
Miklos Szeredi <miklos@xxxxxxxxxx> wrote:

> Jeff Layton <jlayton@xxxxxxxxxx> writes:
> >>
> >> Won't something like fstatat(AT_FDCWD, "", &stat, AT_EMPTY_PATH) risk
> >> looping forever there, or am I missing something?
> >>
> >
> > To make sure I understand, that should be "shortcut" for a lookup of the
> > cwd?
> >
> > So I guess the concern is that you'd do the above and get a successful
> > lookup since you're just going to get back the cwd. At that point,
> > you'd attempt the getattr and get ESTALE back. Then, you'd redo the
> > lookup with LOOKUP_REVAL set -- but since we're operating on the
> > cwd, we don't have a way to redo the lookup since we don't have a
> > pathname that we can look up again...
> >
> > So yeah, I guess if you're sitting in a stale directory, something like
> > that could loop eternally.
> >
> > Do you think the proposed check for fatal_signal_pending is enough to
> > mitigate such a problem? Or do we need to limit the number of retries
> > to address those sorts of loops?
> Lets step back a bit.
> The retry is needed when when we discover during ->getattr() that the
> cached lookup returned a stale file handle.
> If the lookup wasn't cached or if there was no lookup at all
> (stat(".") and friends) then retrying will not gain anything.

That's not necessarily the case, at least not with NFS. It's easily
possible for you to do a full-fledged lookup over the wire, and then
for that inode to be removed prior to issuing a call against the FH that
you got back.

> And that also means that retrying multiple times is pointless, since
> after the first retry we are sure to have up-to-date attributes.

Again, it's not pointless. It's possible (though somewhat pathological)
for you to hit the race above more than once in the same operation.
Granted, it's an unlikely race but it is possible.

> Unfortunately it's impossible for the filesystem to know whether a
> ->getattr (or other inode operation) was perfromed after a cached or a
> non-cached lookup.
> I'm not sure what the right interface for this would be. One would be
> to just pass the "cached-or-not" information as a flag. That works for
> getattr() but not for other operations.
> Another is to introduce atomic lookup+foo variants of these operations
> just like for open. E.g. the lookup+getattr is called if the cached
> lookup fails or if the cached lookup succeeds and the plain ->getattr
> call returns ESTALE.

To do that would require protocol support that we simply don't have. We
don't have a way to (for instance) say via NFS "give me the attributes
for this filename". Well, at least not for NFSv3...

With v4 you could theoretically construct a compound that does that,
but you'd have to assume that the server won't release the reference to
the inode midway through the compound. That's a reasonably safe

While it's nice to consider new atomic ops like this, it's not really
possible with earlier versions of NFS.

Jeff Layton <jlayton@xxxxxxxxxx>
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