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? --b. > > > Surely the change number must > > change no sooner than the change itself is visible, otherwise stale data > > could be cached indefinitely. > > > > If currently implementations behave this way, surely they are broken. > > It's certainly not ideal but we've never been able to offer truly atomic > behavior here given that Linux is a general-purpose OS. The behavior is > a little inconsistent too: > > The c/mtime update and i_version bump on directories (mostly) occur > after the operation. c/mtime updates for files however are mostly driven > by calls to file_update_time, which happens before data is copied to the > pagecache. > > It's not clear to me why it's done this way. Maybe to ensure that the > metadata is up to date in the event that a statx comes in? Improving > this would be nice, but I don't see a way to do that without regressing > performance. > -- > Jeff Layton <jlayton@xxxxxxxxxx>
