Re: [RFC PATCH v5 1/2] hazptr: Implement Hazard Pointers

[Mathieu Desnoyers]

On 2026-06-26 19:06, Paul E. McKenney wrote:
> [ Trimming CC. ]
>
> On Mon, Feb 23, 2026 at 03:44:17PM -0500, Mathieu Desnoyers wrote:
> > This API provides existence guarantees of objects through Hazard
> > Pointers [1] (hazptr).
> >
> > Its main benefit over RCU is that it allows fast reclaim of
> > HP-protected pointers without needing to wait for a grace period.
> >
> > This implementation has 4 statically allocated hazard pointer slots per
> > cpu for the fast path, and relies on a on-stack backup slot allocated by
> > the hazard pointer user as fallback in case no per-cpu slot is
> > available.
> >
> > It integrates with the scheduler to migrate per-CPU slots to the backup
> > slot on context switch. This ensures that the per-CPU slots won't be
> > used by blocked or preempted tasks holding on hazard pointers for a long
> > time.
> >
> > References:
> >
> > [1]: M. M. Michael, "Hazard pointers: safe memory reclamation for
> >       lock-free objects," in IEEE Transactions on Parallel and
> >       Distributed Systems, vol. 15, no. 6, pp. 491-504, June 2004
>
> I (finally) put together a stress test for this, which is the following
> commits on -rcu:
>
> bfb239e036d616 torture: Add a hazptrtorture.c torture test
> adda38c3cffcd6 hazptrtorture: Add testing of on-stack hazptr_ctx structures
> 311e199e482873 hazptrtorture: Add microsecond-scale sleep in readers
> 21ccb41f79a45b hazptrtorture: Enable system-independent CPU overcommit
> 82e8c141265c81 torture: Add a stutter_will_wait() function
> 9ad8169bd3a367 hazptrtorture: Use mnemonic local variables for context information
> cd541d5eb7347f hazptrtorture: Split hazptr_torture_reader_tail() from hazptr_torture_reader()
> 0de8bda6b00051 hazptrtorture: Add kthread to release deferred hazard pointers
> 3deefef00acb22 hazptrtorture: Defer release of hazard pointers
> 65b2cf3c6ea34e hazptrtorture: Add irq_acquire to acquire hazptr from irq
> 98d334c04b58fe hazptrtorture: Use task_state_to_char() for task-state reporting
> 1e89aae50a3fde hazptrtorture: Pass hazptr_pending to hazptr_torture_reader_tail()
> 4b1a177190e2e1 hazptrtorture: Add the ability to disable the writer kthread
> 2c966f58358329 hazptrtorture: Add irq_release to release hazptr from irq
> 16258269b41ed3 hazptrtorture: Accumulate operation statistics
> c090e2659abcb3 doc: Add hazptrtorture module parameters
>
> Initial runs suggest that hazard pointers cannot be acquired by one
> task and released by another.  Is that expected behavior, or is my test
> improperly passing the hazard pointers?  If the latter, we should of
> course document the proper hazard-pointer-passing mechanism.

This is unexpected. I've looked at your test code quickly and could
not figure out what's wrong though. But it's late. How do you observe
that it fails ?

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com