Hello, David. On Tue, Sep 05, 2017 at 03:50:16PM +0100, David Howells wrote: > With one of my latest patches to AFS, there's a set of cell records, where > each cell has a manager work item that mainains that cell, including > refreshing DNS records and excising expired records from the list. Performing > the excision in the manager work item makes handling the fscache index cookie > easier (you can't have two cookies attached to the same object), amongst other > things. > > There's also an overseer work item that maintains a single expiry timer for > all the cells and queues the per-cell work items to do DNS updates and cell > removal. > > The reason that the overseer exists is that it makes it easier to do a put on > a cell. The cell decrements the cell refcount and then wants to schedule the > cell for destruction - but it's no longer permitted to touch the cell. I > could use atomic_dec_and_lock(), but that's messy. It's cleaner just to set > the timer on the overseer and leave it to that. > > However, if someone does rmmod, I have to be able to clean everything up. The > overseer timer may be queued or running; the overseer may be queued *and* > running and may get queued again by the timer; and each cell's work item may > be queued *and* running and may get queued again by the manager. Thanks for the detailed explanation. > > Why can't it be done via the usual "flush from exit"? > > Well, it can, but you need a flush for each separate level of dependencies, > where one dependency will kick off another level of dependency during the > cleanup. > > So what I think I would have to do is set a flag to say that no one is allowed > to set the timer now (this shouldn't happen outside of server or volume cache > clearance), delete the timer synchronously, flush the work queue four times > and then do an RCU barrier. > > However, since I have volumes with dependencies on servers and cells, possibly > with their own managers, I think I may need up to 12 flushes, possibly with > interspersed RCU barriers. Would it be possible to isolate work items for the cell in its own workqueue and use drain_workqueue()? Separating out flush domains is one of the main use cases for dedicated workqueues after all. > It's much simpler to count out the objects than to try and get the flushing > right. I still feel very reluctant to add generic counting & trigger mechanism to work items for this. I think it's too generic a solution for a very specific problem. Thanks. -- tejun
