Re: [PATCH] cpufreq: intel_pstate: Fix for HWP interrupt before driver is ready

[Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>]

On Mon, 2021-09-06 at 10:43 -0600, Jens Axboe wrote:
> On 9/6/21 10:17 AM, Rafael J. Wysocki wrote:
> > On Sat, Sep 4, 2021 at 7:37 AM Srinivas Pandruvada
> > <srinivas.pandruvada@linux.intel.com> wrote:
> > > 
> > > In Lenovo X1 gen9 laptop, HWP interrupts arrive before driver is
> > > ready
> > > to handle on that CPU. Basically didn't even allocated memory for
> > > per
> > > cpu data structure and not even started interrupt enable process
> > > on that
> > > CPU. So interrupt handler observes a NULL pointer to schedule
> > > work.
> > > 
> > > This interrupt was probably for SMM, but since it is redirected
> > > to
> > > OS by OSC call, OS receives it, but not ready to handle. That
> > > redirection
> > > of interrupt to OS was also done to solve one SMM crash on Yoga
> > > 260 for
> > > HWP interrupt a while back.
> > > 
> > > To solve this the HWP interrupt handler should ignore such
> > > request if the
> > > driver is not ready. This will require some flag to wait till the
> > > driver
> > > setup a workqueue to handle on a CPU. We can't simply assume
> > > cpudata to
> > > be NULL and avoid processing as it may not be NULL but data
> > > structure is
> > > not in consistent state.
> > > 
> > > So created a cpumask which sets the CPU on which interrupt was
> > > setup. If
> > > not setup, simply clear the interrupt status and return. Since
> > > the
> > > similar issue can happen during S3 resume, clear the bit during
> > > offline.
> > > 
> > > Since interrupt timing may be before HWP is enabled, use safe MSR
> > > read
> > > writes as before the change for HWP interrupt.
> > > 
> > > Fixes: d0e936adbd22 ("cpufreq: intel_pstate: Process HWP
> > > Guaranteed change notification")
> > > Reported-and-tested-by: Jens Axboe <axboe@kernel.dk>
> > > Signed-off-by: Srinivas Pandruvada < 
> > > srinivas.pandruvada@linux.intel.com>
> > > ---
> > >  drivers/cpufreq/intel_pstate.c | 23 ++++++++++++++++++++++-
> > >  1 file changed, 22 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/cpufreq/intel_pstate.c
> > > b/drivers/cpufreq/intel_pstate.c
> > > index b4ffe6c8a0d0..5ac86bfa1080 100644
> > > --- a/drivers/cpufreq/intel_pstate.c
> > > +++ b/drivers/cpufreq/intel_pstate.c
> > > @@ -298,6 +298,8 @@ static bool hwp_boost __read_mostly;
> > > 
> > >  static struct cpufreq_driver *intel_pstate_driver __read_mostly;
> > > 
> > > +static cpumask_t hwp_intr_enable_mask;
> > > +
> > >  #ifdef CONFIG_ACPI
> > >  static bool acpi_ppc;
> > >  #endif
> > > @@ -1067,11 +1069,15 @@ static void intel_pstate_hwp_set(unsigned
> > > int cpu)
> > >         wrmsrl_on_cpu(cpu, MSR_HWP_REQUEST, value);
> > >  }
> > > 
> > > +static void intel_pstate_disable_hwp_interrupt(struct cpudata
> > > *cpudata);
> > > +
> > >  static void intel_pstate_hwp_offline(struct cpudata *cpu)
> > >  {
> > >         u64 value = READ_ONCE(cpu->hwp_req_cached);
> > >         int min_perf;
> > > 
> > > +       intel_pstate_disable_hwp_interrupt(cpu);
> > > +
> > >         if (boot_cpu_has(X86_FEATURE_HWP_EPP)) {
> > >                 /*
> > >                  * In case the EPP has been set to "performance"
> > > by the
> > > @@ -1645,20 +1651,35 @@ void notify_hwp_interrupt(void)
> > >         if (!hwp_active || !boot_cpu_has(X86_FEATURE_HWP_NOTIFY))
> > >                 return;
> > > 
> > > -       rdmsrl(MSR_HWP_STATUS, value);
> > > +       rdmsrl_safe(MSR_HWP_STATUS, &value);
> > >         if (!(value & 0x01))
> > >                 return;
> > > 
> > > +       if (!cpumask_test_cpu(this_cpu, &hwp_intr_enable_mask)) {
> > > +               wrmsrl_safe(MSR_HWP_STATUS, 0);
> > > +               return;
> > > +       }
> > 
> > Without additional locking, there is a race between this and
> > intel_pstate_disable_hwp_interrupt().
> > 
> > 1. notify_hwp_interrupt() checks hwp_intr_enable_mask() and the
> > target
> > CPU is in there, so it will go for scheduling the delayed work.
> > 2. intel_pstate_disable_hwp_interrupt() runs between the check and
> > the
> > cpudata load below.
> > 3. hwp_notify_work is scheduled on the CPU that isn't there in the
> > mask any more.
> 
> I noticed that too, not clear to me how much of an issue that is in
> practice. But there's definitely a race there.
Glad to see how this is possible from code running in ISR context.

Thanks,
Srinivas

> 
> Can we just revert d0e936adbd22 for now so this can get solved in due
> time? I doubt I'm the only one that got bit by this, but I might be
> the
> only one that bothered to actually figured out what caused my laptop
> to
> crash with a black screen on boot.
>