On Mon, Dec 03, 2018 at 10:16:59AM -0800, Bart Van Assche wrote: > On Mon, 2018-12-03 at 18:32 +0100, Peter Zijlstra wrote: > > On Mon, Dec 03, 2018 at 08:40:48AM -0800, Bart Van Assche wrote: > > > > > > I think we can do this with a free bitmap and an array of 2 pending > > > > bitmaps and an index. Add newly freed entries to the pending bitmap > > > > indicated by the current index, when complete flip the index -- such > > > > that further new bits go to the other pending bitmap -- and call_rcu(). > > > > > > > > Then, on the call_rcu() callback, ie. after a GP has happened, OR our > > > > pending bitmap into the free bitmap, and when the other pending bitmap > > > > isn't empty, flip the index again and start it all again. > > > > > > > > This ensures there is at least one full GP between setting a bit and it > > > > landing in the free mask. > > > > > > Hi Peter, > > > > > > How about the following alternative which requires only two bitmaps > > > instead > > > of three: > > > - Maintain two bitmaps, one for the free entries and one for the entries > > > that are being freed. > > > - Protect all accesses to both bitmaps with the graph lock. > > > - zap_class() sets a bit in the "being freed" bitmap for the entries that > > > should be freed after a GP. > > > - Instead of making free_zapped_classes() wait for a grace period by > > > calling > > > synchronize_sched(), use call_rcu() and do the freeing work from inside > > > the > > > RCU callback. > > > - From inside the RCU callback, set a bit in the "free" bitmap for all > > > entries > > > that have a bit set in the "being freed" bitmap and clears the "being > > > freed" > > > bitmap. > > > > What happens when another unreg happens while the rcu_call thing is > > still pending? > > A new flag will have to keep track of whether or not an RCU callback has > already been scheduled via rcu_call() but not yet executed to avoid double > RCU call complaints.
That's not the only problem there. You either then have to synchronously wait for that flag / rcu_call to complete, or, if you modify the bitmap, ensure it re-queues itself for another GP before committing, which is starvation prone. > In other code a possible alternative would be to > allocate the RCU head data structure dynamically. However, I don't think > that alternative is appropriate inside the lockdep code - I don't want to > introduce a circular dependency between the lockdep code and the memory > allocator. Yes, that's a trainwreck waiting to happen ;-)
