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. > > > > > 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> -- Jeff Layton <jlayton@xxxxxxxxxx>
