>
> 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;
> > +}
> > +
