On Mon, Jul 1, 2024 at 7:50 AM Christian Brauner <brauner@xxxxxxxxxx> wrote: > > > I always thought the rcu delay was to ensure concurrent path walks "see" the > > > > umount not to ensure correct operation of the following mntput()(s). > > > > > > Isn't the sequence of operations roughly, resolve path, lock, deatch, > > release > > > > lock, rcu wait, mntput() subordinate mounts, put path. > > The crucial bit is really that synchronize_rcu_expedited() ensures that > the final mntput() won't happen until path walk leaves RCU mode. > > This allows caller's like legitimize_mnt() which are called with only > the RCU read-lock during lazy path walk to simple check for > MNT_SYNC_UMOUNT and see that the mnt is about to be killed. If they see > that this mount is MNT_SYNC_UMOUNT then they know that the mount won't > be freed until an RCU grace period is up and so they know that they can > simply put the reference count they took _without having to actually > call mntput()_. > > Because if they did have to call mntput() they might end up shutting the > filesystem down instead of umount() and that will cause said EBUSY > errors I mentioned in my earlier mails. But such behaviour could be kept even without an expedited RCU sync. Such as in my alternative patch for this: https://www.spinics.net/lists/linux-fsdevel/msg270117.html I.e. we would still guarantee the final mput is called, but not block the return of the unmount call. -- =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Alexander Larsson Red Hat, Inc alexl@xxxxxxxxxx alexander.larsson@xxxxxxxxx
