Re: [PATCH RFC v3] vfs: make fstatat retry once on ESTALE errors from getattr call

[Jeff Layton <jlayton@redhat.com>]

On Mon, 23 Apr 2012 09:34:12 -0400
"J. Bruce Fields" <bfields@fieldses.org> wrote:

> On Mon, Apr 23, 2012 at 09:12:55AM -0400, Jeff Layton wrote:
> > On Mon, 23 Apr 2012 09:00:09 -0400
> > "J. Bruce Fields" <bfields@fieldses.org> wrote:
> > 
> > > On Mon, Apr 23, 2012 at 08:00:12AM -0400, Jeff Layton wrote:
> > > > On Sun, 22 Apr 2012 07:40:57 +0200
> > > > Miklos Szeredi <miklos@szeredi.hu> wrote:
> > > > 
> > > > > On Fri, Apr 20, 2012 at 11:13 PM, Jeff Layton <jlayton@redhat.com> wrote:
> > > > > > On Fri, 20 Apr 2012 15:37:26 -0500
> > > > > > Malahal Naineni <malahal@us.ibm.com> wrote:
> > > > > >
> > > > > >> Steve Dickson [SteveD@redhat.com] wrote:
> > > > > >> > > 2) if we assume that it is fairly representative of one, how can we
> > > > > >> > > achieve retrying indefinitely with NFS, or at least some large finite
> > > > > >> > > amount?
> > > > > >> > The amount of looping would be peer speculation. If the problem can
> > > > > >> > not be handled by one simple retry I would say we simply pass the
> > > > > >> > error up to the app... Its an application issue...
> > > > > >>
> > > > > >> As someone said, ESTALE is an incorrect errno for a path based call.
> > > > > >> How about turning ESTALE into ENOENT after a retry or few retries?
> > > > > >>
> > > > > >
> > > > > > It's not really the same thing. One could envision an application
> > > > > > that's repeatedly renaming a new file on top of another one. The file
> > > > > > is never missing from the namespace of the server, but you could still
> > > > > > end up getting an ESTALE.
> > > > > >
> > > > > > That would break other atomicity guarantees in an even worse way, IMO...
> > > > > 
> > > > > For directory operations ESTALE *is* equivalent to ENOENT if already
> > > > > retrying with LOOKUP_REVAL.  Think about it.  Atomic replacement by
> > > > > another directory with rename(2) is not an excuse here actually.
> > > > > Local filesystems too can end up with IS_DEAD directory after lookup
> > > > > in that case.
> > > > > 
> > > > 
> > > > Doesn't that violate POSIX? rename(2) is supposed to be atomic, and I
> > > > can't see where there's any exception for that for directories.
> > > 
> > > Hm, but that only allows atomic replacement of the last component of a
> > > path.
> > > 
> > > Suppose you're looking up a path, you've so far reached intermediate
> > > directory "D", and the next step of the lookup (of some entry in D)
> > > returns ESTALE.  Then either:
> > > 
> > > 	- D has since been unlinked, and ENOENT is obviously right.
> > > 	- D was unlinked and then replaced by something else, in which
> > > 	  case there was still a moment when ENOENT was correct.
> > > 	- D was replaced atomically by a rename.  But for the rename to
> > > 	  work it must have been replacing an empty directory, so there
> > > 	  was still a moment when ENOENT would have been correct.
> > 
> > I don't think so...D should always exist in the namespace, so ENOENT
> > would not be correct.
> 
> The operation above is a lookup in D, not a lookup of D.
>
> > Just because it was empty doesn't mean that it
> > didn't exist...
> > 
> > > 	  (Exception: if D was actually a regular file or some other
> > > 	  non-directory object, then ENOTDIR would be the right error:
> > > 	  but if you're able to get at least object type atomically with
> > > 	  a lookup, then you should have noticed this already on lookup
> > > 	  of D.)
> > > 
> > > I think that's what Miklos meant?
> > > 
> > > --b.
> > 
> > Here's an example -- suppose we have two directories: /foo
> > and /bar. /bar is empty. We call:
> > 
> >     rename("/foo","/bar");
> > 
> > ...and at the same time, someone is calling:
> > 
> >     stat("/bar");
> > 
> > ...the calls race and in this condition the stat() gets ESTALE back
> > -- /bar got replaced after we did the lookup.
> > 
> > According to POSIX, the name "/bar" should never be absent from the
> > namespace in this situation, so I'm not sure I understand why returning
> > ENOENT here would be acceptable.
> 
> Yes, agreed, my assertion was just that an ESTALE on a lookup of a
> non-final component is probably equivalent to ENOENT.
> 
> I'm not sure if that's what Miklos meant.
> 

Ahh ok, sorry I misunderstood. Yeah in that case I suppose it would
be ok to replace ESTALE with ENOENT. Ok, so to illustrate...

Suppose we're trying to stat("/bar/baz") instead in the above example.
Then we could just return ENOENT instead on an ESTALE return for the
reasons that Bruce outlined. If the dir was stale, then there was a
at least one point in time where we *know* that "baz" didn't exist.

That doesn't seem like it'll work as a general solution though since it
wouldn't apply to an ESTALE on the last component. For that we'd need
to do something different -- retry the operation in some form, but it
might be potential optimization in the path walking code to avoid
retrying in some cases.

-- 
Jeff Layton <jlayton@redhat.com>