Re: [PATCH bpf-next v1] selftests/bpf: Add ABBCCA case for rqspinlock stress test

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2025-10-24 at 09:42 -0700, Amery Hung wrote:
> On Wed, Oct 22, 2025 at 3:04 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
> > 
> > On Wed, 2025-10-22 at 17:54 +0000, Kumar Kartikeya Dwivedi wrote:
> > > Introduce a new mode for the rqspinlock stress test that exercises a
> > > deadlock that won't be detected by the AA and ABBA checks, such that we
> > > always reliably trigger the timeout fallback. We need 4 CPUs for this
> > > particular case, as CPU 0 is untouched, and three participant CPUs for
> > > triggering the ABBCCA case.
> > > 
> > > Refactor the lock acquisition paths in the module to better reflect the
> > > three modes and choose the right lock depending on the context.
> > > 
> > > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> > > ---
> > 
> > The overhaul makes sense to me and the code is easy to follow.
> > The only nit I have is that test does not fail if deadlock is not detected.
> > E.g. if I remove raw_res_spin_unlock_irqrestore() call in nmi_cb(),
> > there are stall splats in dmesg, but test harness reports success.
> > I suggest adding some signal that all kthreads terminated successfully.
> > 
> > Acked-by: Eduard Zingerman <eddyz87@gmail.com>
> > 
> 
> Maybe it should be another way around? The test must and should have
> triggered deadlocks, so if we count how many times the return of
> raw_res_spin_lock_irqrestore == -EDEADLK or -EITMEDOUT, the number
> should be non-zero.

+1, that would be a good thing to check.

> 
> The test looks good to me otherwise.
> 
> Reviewed-by: Amery Hung <ameryhung@gmail.com>