Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field

NeilBrown
2022-09-08T00:44:47Z

On Wed, 07 Sep 2022, Jan Kara wrote:
> On Wed 07-09-22 09:12:34, Jeff Layton wrote:
> > On Wed, 2022-09-07 at 08:52 -0400, J. Bruce Fields wrote:
> > > On Wed, Sep 07, 2022 at 08:47:20AM -0400, Jeff Layton wrote:
> > > > On Wed, 2022-09-07 at 21:37 +1000, NeilBrown wrote:
> > > > > On Wed, 07 Sep 2022, Jeff Layton wrote:
> > > > > > +The change to \fIstatx.stx_ino_version\fP is not atomic with respect to the
> > > > > > +other changes in the inode. On a write, for instance, the i_version it usually
> > > > > > +incremented before the data is copied into the pagecache. Therefore it is
> > > > > > +possible to see a new i_version value while a read still shows the old data.
> > > > >
> > > > > Doesn't that make the value useless?
> > > > >
> > > >
> > > > No, I don't think so. It's only really useful for comparing to an older
> > > > sample anyway. If you do "statx; read; statx" and the value hasn't
> > > > changed, then you know that things are stable.
> > >
> > > I don't see how that helps. It's still possible to get:
> > >
> > > reader writer
> > > ------ ------
> > > i_version++
> > > statx
> > > read
> > > statx
> > > update page cache
> > >
> > > right?
> > >
> >
> > Yeah, I suppose so -- the statx wouldn't necessitate any locking. In
> > that case, maybe this is useless then other than for testing purposes
> > and userland NFS servers.
> >
> > Would it be better to not consume a statx field with this if so? What
> > could we use as an alternate interface? ioctl? Some sort of global
> > virtual xattr? It does need to be something per-inode.
>
> I was thinking how hard would it be to increment i_version after updating
> data but it will be rather hairy. In particular because of stuff like
> IOCB_NOWAIT support which needs to bail if i_version update is needed. So
> yeah, I don't think there's an easy way how to provide useful i_version for
> general purpose use.
>

Why cannot IOCB_NOWAIT update i_version? Do we not want to wait on the
cmp_xchg loop in inode_maybe_inc_iversion(), or do we not want to
trigger an inode update?

The first seems unlikely, but the second seems unreasonable. We already
acknowledge that after a crash iversion might go backwards and/or miss
changes.

Thanks,
NeilBrown