Re: PATCH: Fix race in cpu_down (hotplug cpu)

[Shaohua Li <shaohua.li@intel.com>]

On Mon, 2005-09-19 at 11:53 +0530, Srivatsa Vaddagiri wrote:
> On Mon, Sep 19, 2005 at 04:11:11PM +1000, Nigel Cunningham wrote:
> > > Ok, that makes sense. Nigel, could you confirm which idle routine you are 
> > > using?
> > 
> > >From dmesg:
> > 
> > monitor/mwait feature present.
> > using mwait in idle threads.
> 
> Ok, that may explain why __cpu_die is timing out for you! Can you try a 
> (untested, I am afraid) patch on these lines: 
> 
> --- process.c.org	2005-09-19 11:44:57.000000000 +0530
> +++ process.c	2005-09-19 11:48:28.000000000 +0530
> @@ -245,16 +245,18 @@ EXPORT_SYMBOL_GPL(cpu_idle_wait);
>   */
>  static void mwait_idle(void)
>  {
> +	int cpu = raw_smp_processor_id();
> +
>  	local_irq_enable();
>  
>  	if (!need_resched()) {
>  		set_thread_flag(TIF_POLLING_NRFLAG);
>  		do {
>  			__monitor((void *)&current_thread_info()->flags, 0, 0);
> -			if (need_resched())
> +			if (need_resched() || cpu_is_offline(cpu))
>  				break;
if the breakpoint is here, you will still have trouble.

>  			__mwait(0, 0);
> -		} while (!need_resched());
> +		} while (!need_resched() || !cpu_is_offline(cpu));
>  		clear_thread_flag(TIF_POLLING_NRFLAG);
>  	}
>  }
> 
> 
> Other idle routines will need similar fix.
> 
> > Ok, but what about default_idle?
> 
> default_idle should be fine as it is. IOW it should not cause __cpu_die to 
> timeout.
Why default_idle should be fine? it can be preempted before the
'local_irq_disable' check. Even with Nigel's patch, there is a very
small window at safe_halt (after 'sti' but before 'hlt').

Thanks,
Shaohua