Re: [PATCH RFC -rcu/dev] hazptr: Record per-CPU slot owner after claiming the slot

[Andrea Parri <parri.andrea@gmail.com>]

On Tue, Oct 20, 2026 at 02:12:09PM -0700, Paul E. McKenney wrote:

[...]

> Thank you all!  Queued for testing and further review.

Thank you!

> Andrea, on your KUNIT reproducer, is there any chance that this code
> can be folded into the existing hazptrtorture.c work so that it just
> gets tested automatically?

Yes, though not as it is: the KUnit test relies on a test-only hook
in __hazptr_try_acquire(), which I would rather not add to hazptr.h.
hazptrtorture already has the interrupt side, in
hazptr_torture_acquire(), but its readers rarely reach
__hazptr_acquire(), where the race is.

So v1 of the hazptrtorture series will add, as patches 2 and 3:

  - two CONFIG_HAZPTR_DEBUG checks of the recorded slot owner, in
    hazptr_release_debug() and hazptr_note_context_switch(), and

  - a nest_acquire module parameter (default 2) that makes readers
    take a second, nested hazard pointer through __hazptr_acquire().

Without the fix, PREEMPT and NOPREEMPT then report the race about 6
seconds after boot; with nest_acquire=0, they run 10 minutes without
a report.  The nesting costs about 30% of the readers' loop
iterations, so it could instead default to zero and be enabled from
the scenario files, if you prefer.

Thanks,
  Andrea