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Date:   Fri, 1 Feb 2019 18:01:12 +0000
From:   Will Deacon <will.deacon@arm.com>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     "Li, Meng" <Meng.Li@windriver.com>,
        "mark.rutland@arm.com" <mark.rutland@arm.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        suzuki.poulose@arm.com
Subject: Re: Could you please help to have a look a bug trace in pmu arm-cci.c
Message-ID: <20190201180112.GA14755@fuggles.cambridge.arm.com>
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On Wed, Jan 30, 2019 at 07:09:42PM +0000, Robin Murphy wrote:
> 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 */
> 
>         ret = cci_pmu_init(cci_pmu, pdev);

So at this point we've registered the PMU with perf, so I think we're open
to userspace. Given that things like pmu_cpumask_attr_show() call
cpumask_of(cci_pmu->cpu), having a cpu of -1 seems like a bad idea.

Why not just use the _cpuslocked() notifier registration functions so that
we don't need to disable preemption?

Will