Re: [PATCH] lockdep: Speed up lockdep_unregister_key() with expedited RCU synchronization

Paul E. McKenney
2025-03-31T18:33:16Z

On Mon, Mar 31, 2025 at 01:33:22PM -0400, Waiman Long wrote:
> On 3/31/25 1:26 PM, Boqun Feng wrote:
> > On Wed, Mar 26, 2025 at 11:39:49AM -0400, Waiman Long wrote:
> > [...]
> > > > > Anyway, that may work. The only problem that I see is the issue of nesting
> > > > > of an interrupt context on top of a task context. It is possible that the
> > > > > first use of a raw_spinlock may happen in an interrupt context. If the
> > > > > interrupt happens when the task has set the hazard pointer and iterating the
> > > > > hash list, the value of the hazard pointer may be overwritten. Alternatively
> > > > > we could have multiple slots for the hazard pointer, but that will make the
> > > > > code more complicated. Or we could disable interrupt before setting the
> > > > > hazard pointer.
> > > > Or we can use lockdep_recursion:
> > > >
> > > > preempt_disable();
> > > > lockdep_recursion_inc();
> > > > barrier();
> > > >
> > > > WRITE_ONCE(*hazptr, ...);
> > > >
> > > > , it should prevent the re-entrant of lockdep in irq.
> > > That will probably work. Or we can disable irq. I am fine with both.
> > Disabling irq may not work in this case, because an NMI can also happen
> > and call register_lock_class().
> Right, disabling irq doesn't work with NMI. So incrementing the recursion
> count is likely the way to go and I think it will work even in the NMI case.
>
> >
> > I'm experimenting a new idea here, it might be better (for general
> > cases), and this has the similar spirit that we could move the
> > protection scope of a hazard pointer from a key to a hash_list: we can
> > introduce a wildcard address, and whenever we do a synchronize_hazptr(),
> > if the hazptr slot equal to wildcard, we treat as it matches to any ptr,
> > hence synchronize_hazptr() will still wait until it's zero'd. Not only
> > this could help in the nesting case, it can also be used if the users
> > want to protect multiple things with this simple hazard pointer
> > implementation.
>
> I think it is a good idea to add a wildcard for the general use case.
> Setting the hazptr to the list head will be enough for this particular case.

Careful! If we enable use of wildcards outside of the special case
of synchronize_hazptr(), we give up the small-memory-footprint advantages
of hazard pointers. You end up having to wait on all hazard-pointer
readers, which was exactly why RCU was troublesome here. ;-)

Thanx, Paul