Subject: Re: [PATCH][RFC] preempt_count corruption across H_CEDE call with CONFIG_PREEMPT on pseries

From: Darren Hart <[email-masked]>
Date: Wed, 01 Sep 2010 13:42:16 -0700

On 09/01/2010 12:59 PM, Steven Rostedt wrote:
> On Wed, 2010-09-01 at 11:47 -0700, Darren Hart wrote:
> 
>> from tip/rt/2.6.33 causes the preempt_count() to change across the cede
>> call.  This patch appears to prevents the proxy preempt_count assignment
>> from happening. This non-local-cpu assignment to 0 would cause an
>> underrun of preempt_count() if the local-cpu had disabled preemption
>> prior to the assignment and then later tried to enable it. This appears
>> to be the case with the stack of __trace_hcall* calls preceeding the
>> return from extended_cede_processor() in the latency format trace-cmd
>> report:
>>
>>   <idle>-0       1d....   201.252737: function:             .cpu_die
> 
> Note, the above 1d.... is a series of values. The first being the CPU,
> the next if interrupts are disabled, the next if the NEED_RESCHED flag
> is set, the next is softirqs enabled or disabled, next the
> preempt_count, and finally the lockdepth count.
> 
> Here we only care about the preempt_count, which is zero when '.' and a
> number if it is something else. It is the second to last field in that
> list.
> 
> 
>>   <idle>-0       1d....   201.252738: function:                .pseries_mach_cpu_die
>>   <idle>-0       1d....   201.252740: function:                   .idle_task_exit
>>   <idle>-0       1d....   201.252741: function:                      .switch_slb
>>   <idle>-0       1d....   201.252742: function:                   .xics_teardown_cpu
>>   <idle>-0       1d....   201.252743: function:                      .xics_set_cpu_priority
>>   <idle>-0       1d....   201.252744: function:             .__trace_hcall_entry
>>   <idle>-0       1d..1.   201.252745: function:                .probe_hcall_entry
> 
>                        ^
>                 preempt_count set to 1
> 
>>   <idle>-0       1d..1.   201.252746: function:             .__trace_hcall_exit
>>   <idle>-0       1d..2.   201.252747: function:                .probe_hcall_exit
>>   <idle>-0       1d....   201.252748: function:             .__trace_hcall_entry
>>   <idle>-0       1d..1.   201.252748: function:                .probe_hcall_entry
>>   <idle>-0       1d..1.   201.252750: function:             .__trace_hcall_exit
>>   <idle>-0       1d..2.   201.252751: function:                .probe_hcall_exit
>>   <idle>-0       1d....   201.252752: function:             .__trace_hcall_entry
>>   <idle>-0       1d..1.   201.252753: function:                .probe_hcall_entry
>                    ^   ^
>                   CPU  preempt_count
> 
> Entering the function probe_hcall_entry() the preempt_count is 1 (see
> below). But probe_hcall_entry does:
> 
> 	h = &get_cpu_var(hcall_stats)[opcode / 4];
> 
> Without doing the put (which it does in probe_hcall_exit())
> 
> So exiting the probe_hcall_entry() the prempt_count is 2.
> The trace_hcall_entry() will do a preempt_enable() making it leave as 1.
> 
> 
>>   offon.sh-3684  6.....   201.466488: bprint:               .smp_pSeries_kick_cpu : resetting pcnt to 0 for cpu 1
> 
> This is CPU 6, changing the preempt count from 1 to 0.
> 
>>
>> preempt_count() is reset from 1 to 0 by smp_startup_cpu() without the
>> QCSS_NOT_STOPPED check from the patch above.
>>
>>   <idle>-0       1d....   201.466503: function:             .__trace_hcall_exit
> 
> Note: __trace_hcall_exit() and __trace_hcall_entry() basically do:
> 
>  preempt_disable();
>  call probe();
>  preempt_enable();
> 
> 
>>   <idle>-0       1d..1.   201.466505: function:                .probe_hcall_exit
> 
> The preempt_count of 1 entering the probe_hcall_exit() is because of the
> preempt_disable() shown above. It should have been entered as a 2.
> 
> But then it does:
> 
> 
> 	put_cpu_var(hcall_stats);
> 
> making preempt_count 0.
> 
> But the preempt_enable() in the trace_hcall_exit() causes this to be -1.
> 
> 
>>   <idle>-0       1d.Hff.   201.466507: bprint:               .pseries_mach_cpu_die : after cede: ffffffff
>>
>> With the preempt_count() being one less than it should be, the final
>> preempt_enable() in the trace_hcall path drops preempt_count to
>> 0xffffffff, which of course is an illegal value and leads to a crash.
> 
> I'm confused to how this works in mainline?

Turns out it didn't. 2.6.33.5 with CONFIG_PREEMPT=y sees this exact same
behavior. The following, part of the 2.6.33.6 stable release, prevents
this from happening:

aef40e87d866355ffd279ab21021de733242d0d5
powerpc/pseries: Only call start-cpu when a CPU is stopped

--- a/arch/powerpc/platforms/pseries/smp.c
+++ b/arch/powerpc/platforms/pseries/smp.c
@@ -82,6 +82,12 @@ static inline int __devinit smp_startup_cpu(unsigned
int lcpu)

        pcpu = get_hard_smp_processor_id(lcpu);

+       /* Check to see if the CPU out of FW already for kexec */
+       if (smp_query_cpu_stopped(pcpu) == QCSS_NOT_STOPPED){
+               cpu_set(lcpu, of_spin_map);
+               return 1;
+       }
+
        /* Fixup atomic count: it exited inside IRQ handler. */
        task_thread_info(paca[lcpu].__current)->preempt_count   = 0;

The question is now, Is this the right fix? If so, perhaps we can update
the comment to be a bit more clear and not refer solely to kexec.

Michael Neuling, can you offer any thoughts here? We hit this EVERY
TIME, which makes me wonder if the offline/online path could do this
without calling smp_startup_cpu at all.

-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team