linux-ext4 - Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO

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Date:   Wed, 7 Sep 2022 08:20:33 -0400
From:   bfields@...ldses.org (J. Bruce Fields)
To:     NeilBrown <neilb@...e.de>
Cc:     Jeff Layton <jlayton@...nel.org>, tytso@....edu,
        adilger.kernel@...ger.ca, djwong@...nel.org, david@...morbit.com,
        trondmy@...merspace.com, viro@...iv.linux.org.uk,
        zohar@...ux.ibm.com, xiubli@...hat.com, chuck.lever@...cle.com,
        lczerner@...hat.com, jack@...e.cz, brauner@...nel.org,
        fweimer@...hat.com, linux-man@...r.kernel.org,
        linux-api@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        ceph-devel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

On Wed, Sep 07, 2022 at 09:37:33PM +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?  Surely the change number must
> change no sooner than the change itself is visible, otherwise stale data
> could be cached indefinitely.

For the purposes of NFS close-to-open, I guess all we need is for the
change attribute increment to happen sometime between the open and the
close.

But, yes, it'd seem a lot more useful if it was guaranteed to happen
after.  (Or before and after both--extraneous increments aren't a big
problem here.)

--b.

> 
> If currently implementations behave this way, surely they are broken.
> 
> NeilBrown