On Mon, Jul 01, 2024 at 02:10:31PM GMT, Christian Brauner wrote: > On Mon, Jul 01, 2024 at 10:41:40AM GMT, Alexander Larsson wrote: > > 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. > > That's fine but the patch as sent doesn't work is my point. It'll cause > exactly the issues described earlier, no? So I'm confused why this > version simply ended up removing synchronize_rcu_expedited() when > the proposed soluton seems to have been to use queue_rcu_work(). > > But anyway, my concern with this is still that this changes the way > MNT_DETACH behaves when you shut down a non-busy filesystem with > MNT_DETACH as outlined in my other mail. > > If you find a workable version I'm not entirely opposed to try this but > I wouldn't be surprised if this causes user visible issues for anyone > that uses MNT_DETACH on a non-used filesystem. Correction: I misremembered that umount_tree() is called with UMOUNT_SYNC only in the case that umount() isn't called with MNT_DETACH. I mentioned this yesterday in the thread but just in case you missed it I want to spell it out in detail as well. This is relevant because UMOUNT_SYNC will raise MNT_SYNC_UMOUNT on all mounts it unmounts. And that ends up being checked in legitimize_mnt() to ensure that legitimize_mnt() doesn't call mntput() during path lookup and risking EBUSY for a umount(..., 0) + mount() sequence for the same filesystem. But for umount(.., MNT_DETACH) UMOUNT_SYNC isn't passed and so MNT_SYNC_UMOUNT isn't raised on the mount and so legitimize_mnt() may end up doing the last mntput() and cleaning up the filesystem. In other words, a umount(..., MNT_DETACH) caller needs to be prepared to deal with EBUSY for a umount(..., MNT_DETACH) + mount() sequence. So I think we can certainly try this as long as we make it via queue_rcu_work() to handle the other mntput_no_expire() grace period dependency we discussed upthread. Thanks for making take a closer look.
