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.desnoy...@efficios.com/
Regards,
Boqun
