Re: [PATCH 0/8] Introduce simple hazard pointers for lockdep

[Paul E. McKenney]

On Thu, Jun 26, 2025 at 08:47:25AM -0700, Boqun Feng wrote:
> On Thu, Jun 26, 2025 at 03:16:49AM -0700, Christoph Hellwig wrote:
> > On Wed, Jun 25, 2025 at 07:08:57AM -0700, Boqun Feng wrote:
> > > Sure, I will put one for the future version, here is the gist:
> > 
> > Thanks a lot!
> > 
> > > The updater's wait can finish immediately if no one is accessing 'a', in
> > > other words it doesn't need to wait for reader 2.
> > 
> > So basically it is the RCU concept, but limited to protecting exactly
> > one pointer update per critical section with no ability for the read
> > to e.g. acquire a refcount on the objected pointed to by that pointer?
> 
> For the current simple hazard pointer, yes. But simple hazard pointers
> is easily to extend so support reading:
> 
> 	{ gp is a global pointer }
> 
> 	Reader				Updater
> 	======				=======
> 	g = shazptr_acquire(p):
> 	      WRITE_ONCE(*this_cpu_ptr(slot), gp);
> 	      smp_mb();
> 	
> 	if (READ_ONCE(gp) == *this_cpu_ptr(slot)) {
> 	    // still being protected.
> 	    <can read gp here>
> 					to_free = READ_ONCE(gp);
> 					WRITE_ONCE(gp, new);
> 					synchronize_shazptr(to_free):
> 					  smp_mb();
> 					  // wait on the slot of reader
> 					  // CPU being 0.
> 					  READ_ONCE(per_cpu(reader, slot));
> 	}
> 
> 	shazptr_clear(g):
> 	  WRITE_ONCE(*this_cpu_ptr(slot), NULL); // unblock synchronize_shazptr()
> 
> 
> Usually the shazptr_acqurie() + "pointer comparison"* is called
> shazptr_try_protect().
> 
> I will add a document about this in the next version along with other
> bits of hazard pointers.
> 
> [*]: The pointer comparison is more complicated topic, but Mathieu has
>      figured out how to do it correctly:
> 
>      https://lore.kernel.org/lkml/20241008135034.1982519-2-mathieu.desnoyers@xxxxxxxxxxxx/

It might be helpful to add that, at a high level, hazard pointers
are a scalable replacement for reference counting.  At a similarly
high level, RCU is a scalable replacement for reader-writer locking.
At lower levels, there is considerable overlap in applicability, so that
you can use RCU to replace many reference-counting use cases and hazard
pointers to replace many reader-writer-locking use cases..

Plus, as both Mathieu and Boqun pointed out, both RCU and hazard pointers
can be combined with other synchronization mechanisms, including each
other.

							Thanx, Paul