Re: [PATCH AUTOSEL 4.9 4/4] thermal: intel_powerclamp: Use get_cpu() instead of smp_processor_id() to avoid crash

[Chen Yu <yu.c.chen@intel.com>]

Hi Pavel,
On 2022-10-11 at 13:36:46 +0200, Pavel Machek wrote:
> Hi!
> 
> > From: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> > 
> > [ Upstream commit 68b99e94a4a2db6ba9b31fe0485e057b9354a640 ]
> > 
> > When CPU 0 is offline and intel_powerclamp is used to inject
> > idle, it generates kernel BUG:
> > 
> > BUG: using smp_processor_id() in preemptible [00000000] code: bash/15687
> > caller is debug_smp_processor_id+0x17/0x20
> > CPU: 4 PID: 15687 Comm: bash Not tainted 5.19.0-rc7+ #57
> > Call Trace:
> > <TASK>
> > dump_stack_lvl+0x49/0x63
> > dump_stack+0x10/0x16
> > check_preemption_disabled+0xdd/0xe0
> > debug_smp_processor_id+0x17/0x20
> > powerclamp_set_cur_state+0x7f/0xf9 [intel_powerclamp]
> > ...
> > ...
> > 
> > Here CPU 0 is the control CPU by default and changed to the current CPU,
> > if CPU 0 offlined. This check has to be performed under cpus_read_lock(),
> > hence the above warning.
> > 
> > Use get_cpu() instead of smp_processor_id() to avoid this BUG.
> 
> This has exactly the same problem as smp_processor_id(), you just
> worked around the warning. If it is okay that control_cpu contains
> stale value, could we have a comment explaining why?
>
May I know why does control_cpu have stale value? The control_cpu
is a random picked online CPU which will be used later to collect statistics.
As long as the control_cpu is online, it is valid IMO.

thanks,
Chenyu
> Thanks,
> 								Pavel
> 								
> > +++ b/drivers/thermal/intel_powerclamp.c
> > @@ -519,8 +519,10 @@ static int start_power_clamp(void)
> >  
> >  	/* prefer BSP */
> >  	control_cpu = 0;
> > -	if (!cpu_online(control_cpu))
> > -		control_cpu = smp_processor_id();
> > +	if (!cpu_online(control_cpu)) {
> > +		control_cpu = get_cpu();
> > +		put_cpu();
> > +	}
> >  
> >  	clamping = true;
> >  	schedule_delayed_work(&poll_pkg_cstate_work, 0);
> > -- 
> > 2.35.1
> 
> -- 
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