Re: [PATCH 22/27] locking/lockdep: Reuse list entries that are no longer in use

[Peter Zijlstra <peterz@infradead.org>]

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?