Re: [PATCH v7 06/10] tracing/osnoise: Reset cleanup

[Crystal Wood <crwood@redhat.com>]

On Fri, 2026-11-13 at 07:20 +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential
> issue to consider:
> - [Low] tracing/osnoise: CPU hotplug race in timerlat_fd_open() leaves
>   stale fdd
> --
> 
> commit 25ee38a6983ff50cb9227e203eed4ba8758cefa4
> Author: Crystal Wood <crwood@redhat.com>
> 
> tracing/osnoise: Reset cleanup
> 
> This commit reorganizes per-CPU and global state resets in the osnoise and
> timerlat tracers when stopping kthreads, detaching file descriptors, or
> restarting the tracer.
> 
> > diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
> > index e61f57230eefb..8db0697b69e4a 100644
> > --- a/kernel/trace/trace_osnoise.c
> > +++ b/kernel/trace/trace_osnoise.c
> [ ... ]
> > @@ -408,6 +380,39 @@ static struct osnoise_data {
> >  #endif
> >  };
> >  
> > +static inline void osn_reset_cpu(int cpu)
> > +{
> > +	struct osnoise_variables *osn = cpu_osn_var(cpu);
> > +
> > +	WARN_ON_ONCE(osn->kthread);
> > +	WARN_ON_ONCE(osn->fdd);
> > +
> > +	osn->sampling = false;
> > +	osn->pid = 0;
> > +	tlat_reset_cpu(cpu);
> > +}
> > +
> > +/*
> > + * osn_var_reset - Reset non-per-cpu state, and check for stale data
> > + */
> > +static inline void osn_var_reset(void)
> > +{
> > +	struct osnoise_variables *osn;
> > +	int cpu;
> > +
> > +	for_each_possible_cpu(cpu) {
> > +		osn = cpu_osn_var(cpu);
> > +
> > +		WARN_ON_ONCE(osn->kthread);
> > +		WARN_ON_ONCE(osn->fdd);
> > +		WARN_ON_ONCE(osn->sampling);
> > +		memset(&osn->zero, 0, sizeof(osn->zero));
> > +	}
> > +
> > +	tlat_var_reset();
> > +	osnoise_data.tainted = false;
> > +}
> 
> [Severity: Low]
> Can osn_var_reset() hit WARN_ON_ONCE(osn->fdd) if timerlat_fd_open() races
> with CPU hotplug takedown?
> 
> timerlat_fd_open() disables migration without holding cpus_read_lock() or
> checking whether the CPU is still active under osn_var->lock.  If a task
> pinned to CPU0 enters timerlat_fd_open() just as CPU0 is being offlined,
> balance_push() skips the task while migration is disabled, allowing
> osnoise_cpu_die() to run before timerlat_fd_open() attaches osn_var->fdd:

True, but not a new problem other than that we have a new way of
detecting it.  I can send a followup patch once I return (I'll be away
for the next three weeks).


-Crystal