RE: Could you please help to have a look a bug trace in pmu arm-cci.c

[Li, Meng]

> -----Original Message-----
> From: Robin Murphy [mailto:robin.murphy@xxxxxxx]
> Sent: Thursday, January 31, 2019 3:10 AM
> To: Will Deacon; Li, Meng
> Cc: mark.rutland@xxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; suzuki.poulose@xxxxxxx
> Subject: Re: Could you please help to have a look a bug trace in pmu arm-cci.c
> 
> On 2019-01-30 6:21 pm, Will Deacon wrote:
> > [+Suzuki and Robin]
> >
> > On Mon, Jan 28, 2019 at 07:19:20AM +0000, Li, Meng wrote:
> >> When enable kernel configure CONFIG_DEBUG_ATOMIC_SLEEP, there is
> below trace
> >> during pmu arm cci driver probe phase.
> >>
> >> [ 1.983337] BUG: sleeping function called from invalid context at
> kernel/locking/rtmutex.c:2004
> >> [ 1.983340] in_atomic(): 1, irqs_disabled(): 0, pid: 1, name: swapper/0
> >> [ 1.983342] Preemption disabled at:
> >> [ 1.983353] [<ffffff80089801f4>] cci_pmu_probe+0x1dc/0x488
> >> [ 1.983360] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.18.20-rt8-yocto-
> preempt-rt #1
> >> [ 1.983362] Hardware name: ZynqMP ZCU102 Rev1.0 (DT)
> >> [ 1.983364] Call trace:
> >> [ 1.983369] dump_backtrace+0x0/0x158
> >> [ 1.983372] show_stack+0x24/0x30
> >> [ 1.983378] dump_stack+0x80/0xa4
> >> [ 1.983383] ___might_sleep+0x138/0x160
> >> [ 1.983386] __might_sleep+0x58/0x90
> >> [ 1.983391] __rt_mutex_lock_state+0x30/0xc0
> >> [ 1.983395] _mutex_lock+0x24/0x30
> >> [ 1.983400] perf_pmu_register+0x2c/0x388
> >> [ 1.983404] cci_pmu_probe+0x2bc/0x488
> >> [ 1.983409] platform_drv_probe+0x58/0xa8
> >>
> >> Because get_cpu() is invoked, preempt is disable, finally, trace occurs
> when
> >> call might_sleep()
> >
> > Hmm, the {get,put}_cpu() usage here looks very broken to me. There's the
> > fact that it might sleep, but also the assignment to g_cci_pmu is done after
> > we've re-enabled preemption, so there's a race with CPU hotplug there
> too.
> 
> Hmm, looks like I failed to appreciate that particular race at the time
> - indeed the global should probably be assigned immediately after
> cci_pmu_init() has succeeded.
> 
> > I don't think we can simply register the hotplug notifier before registering
> > the PMU, because we can't call into perf_pmu_migrate_context() until the
> PMU
> > has been registered. Perhaps we need to use the _cpuslocked() versions
> of
> > the hotplug notifier registration functions.
> >
> > I tried looking at some other drivers, but they all look broken to me, so
> > there's a good chance I'm missing something. Anybody know how this is
> > supposed to work?
> 
> As I understand the general pattern, we register the notifier last to
> avoid taking a hotplug callback with a partly-initialised PMU state,
> however since the CPU we've picked is part of that PMU state, we also
> want to avoid getting migrated off that CPU before the notifier is in
> place lest things get out of sync, hence disabling preemption. As far as
> the correctness of implementing that logic, though, it was like that
> when I got here so I've always just assumed it was fine :)
> 
> I guess the question is whether we actually need to pick our nominal CPU
> before perf_pmu_register(), or if something like the below would suffice
> - what do you reckon?
> 
> Robin.
> 
> ----->8-----
> diff --git a/drivers/perf/arm-cci.c b/drivers/perf/arm-cci.c
> index 1bfeb160c5b1..da9309ff80d7 100644
> --- a/drivers/perf/arm-cci.c
> +++ b/drivers/perf/arm-cci.c
> @@ -1692,19 +1692,18 @@ static int cci_pmu_probe(struct platform_device
> *pdev)
>          raw_spin_lock_init(&cci_pmu->hw_events.pmu_lock);
>          mutex_init(&cci_pmu->reserve_mutex);
>          atomic_set(&cci_pmu->active_events, 0);
> -       cci_pmu->cpu = get_cpu();
> +       cci_pmu->cpu = -1; /* Avoid races until hotplug notifier is alive */

If we set cci_pmu->cpu = -1 here, I think we should modify code of cci_pmu_event_init() as below:
//////// original ////////////////////////
	if (event->cpu < 0)
		return -EINVAL;
	event->cpu = cci_pmu->cpu;
//////// original ////////////////////////
Change into
//////// new ////////////////////////
	event->cpu = cci_pmu->cpu;
	if (event->cpu < 0)
		return -EINVAL;
//////// new ////////////////////////
Move this if condition after the assignment value operation.
Otherwise, when call perf_event_open()->find_get_context(), 
int cpu = event->cpu = -1;
-1 as a parameter passed into per_cpu_ptr(), and return a wrong cpuctx,
If we continue to execute code with the wrong cpuctx, kernel may crash.

I am not sure whether my analysis is reasonable, only just for your reference.

Thanks,
Limeng

> 
>          ret = cci_pmu_init(cci_pmu, pdev);
> -       if (ret) {
> -               put_cpu();
> +       if (ret)
>                  return ret;
> -       }
> +       g_cci_pmu = cci_pmu;
> 
>          cpuhp_setup_state_nocalls(CPUHP_AP_PERF_ARM_CCI_ONLINE,
>                                    "perf/arm/cci:online", NULL,
>                                    cci_pmu_offline_cpu);
> -       put_cpu();
> -       g_cci_pmu = cci_pmu;
> +       cci_pmu->cpu = smp_processor_id();
> +
>          pr_info("ARM %s PMU driver probed", cci_pmu->model->name);
>          return 0;
>   }