Subject: Re: [PATCH v10 14/30] tracing: Add a trace remote module for testing

From: Vincent Donnefort <[email-masked]>
Date: Thu, 29 Jan 2026 21:50:16 +0000

> 
> Where was the cpus_read lock taken?

I am pretty sure that's leftover of when I was reading some CPU mask :-(

> > +	cpus_read_unlock();
> > +
> > +	return ERR_PTR(ret);
> > +}
> > +

[...]

> > +static ssize_t
> > +write_event_write(struct file *filp, const char __user *ubuf, size_t cnt, loff_t *pos)
> > +{
> > +	struct remote_event_format_selftest *evt_test;
> > +	struct simple_rb_per_cpu *cpu_buffer;
> > +	unsigned long val;
> > +	int ret;
> > +
> > +	ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
> > +	if (ret)
> > +		return ret;
> > +
> > +	guard(mutex)(&simple_rbs_lock);
> > +
> > +	if (!remote_event_selftest.enabled)
> > +		return -ENODEV;
> > +
> 
> You want a guard(preempt)(); here...
> 
> > +	cpu_buffer = *this_cpu_ptr(&simple_rbs);
> 
> Otherwise this triggers:
> 
>  BUG: using smp_processor_id() in preemptible [00000000] code: bash/1096
>  caller is write_event_write+0xe0/0x230 [remote_test]
> 
> -- Steve

Ha, of course! And also for a per-CPU ring-buffer, that is way better to make
sure we actually do all the writing from the same CPU even though in that case
the simple_rbs_lock protect us.

> 
> > +	if (!cpu_buffer)
> > +		return -ENODEV;
> > +
> > +	evt_test = simple_ring_buffer_reserve(cpu_buffer,
> > +					      sizeof(struct remote_event_format_selftest),
> > +					      trace_clock_global());
> > +	if (!evt_test)
> > +		return -ENODEV;
> > +
> > +	evt_test->hdr.id = REMOTE_TEST_EVENT_ID;
> > +	evt_test->id = val;
> > +
> > +	simple_ring_buffer_commit(cpu_buffer);
> > +
> > +	return cnt;
> > +}
> > +