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

["J. Bruce Fields" <bfields-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org>]

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-sUDqSbJrdHQHWmgEVkV9KA@public.gmane.org> wrote:
> 
> > On Fri, Apr 20, 2012 at 11:13 PM, Jeff Layton <jlayton-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> > > On Fri, 20 Apr 2012 15:37:26 -0500
> > > Malahal Naineni <malahal-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org> wrote:
> > >
> > >> Steve Dickson [SteveD-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org] 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.
	  (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.
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