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SubjectRe: [PATCH] soc: qcom: rpmh-rsc: Remove the pm_lock
From
Date
Hi,

On 4/14/2020 10:53 PM, Douglas Anderson wrote:
> It has been postulated that the pm_lock is bad for performance because
> a CPU currently running rpmh_flush() could block other CPUs from
> coming out of idle. Similarly CPUs coming out of / going into idle
> all need to contend with each other for the spinlock just to update
> the variable tracking who's in PM.
>
> Let's optimize this a bit. Specifically:
>
> - Use a count rather than a bitmask. This is faster to access and
> also means we can use the atomic_inc_return() function to really
> detect who the last one to enter PM was.
> - Accept that it's OK if we race and are doing the flush (because we
> think we're last) while another CPU is coming out of idle. As long
> as we block that CPU if/when it tries to do an active-only transfer
> we're OK.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
> drivers/soc/qcom/rpmh-internal.h | 7 ++---
> drivers/soc/qcom/rpmh-rsc.c | 46 +++++++++++++++-----------------
> 2 files changed, 24 insertions(+), 29 deletions(-)
>
> diff --git a/drivers/soc/qcom/rpmh-internal.h b/drivers/soc/qcom/rpmh-internal.h
> index dba8510c0669..449cd511702b 100644
> --- a/drivers/soc/qcom/rpmh-internal.h
> +++ b/drivers/soc/qcom/rpmh-internal.h
> @@ -97,7 +97,7 @@ struct rpmh_ctrlr {
> * @num_tcs: Number of TCSes in this DRV.
> * @rsc_pm: CPU PM notifier for controller.
> * Used when solver mode is not present.
> - * @cpus_entered_pm: CPU mask for cpus in idle power collapse.
> + * @cpus_in_pm: Number of CPUs not in idle power collapse.
> * Used when solver mode is not present.
> * @tcs: TCS groups.
> * @tcs_in_use: S/W state of the TCS; only set for ACTIVE_ONLY
> @@ -111,8 +111,6 @@ struct rpmh_ctrlr {
> * grabbing this lock and a tcs_lock at the same time,
> * grab the tcs_lock first so we always have a
> * consistent lock ordering.
> - * @pm_lock: Synchronize during PM notifications.
> - * Used when solver mode is not present.
> * @client: Handle to the DRV's client.
> */
> struct rsc_drv {
> @@ -121,11 +119,10 @@ struct rsc_drv {
> int id;
> int num_tcs;
> struct notifier_block rsc_pm;
> - struct cpumask cpus_entered_pm;
> + atomic_t cpus_in_pm;
> struct tcs_group tcs[TCS_TYPE_NR];
> DECLARE_BITMAP(tcs_in_use, MAX_TCS_NR);
> spinlock_t lock;
> - spinlock_t pm_lock;
> struct rpmh_ctrlr client;
> };
>
> diff --git a/drivers/soc/qcom/rpmh-rsc.c b/drivers/soc/qcom/rpmh-rsc.c
> index 732316bb67dc..4e45a8ac6cde 100644
> --- a/drivers/soc/qcom/rpmh-rsc.c
> +++ b/drivers/soc/qcom/rpmh-rsc.c
> @@ -740,6 +740,8 @@ int rpmh_rsc_write_ctrl_data(struct rsc_drv *drv, const struct tcs_request *msg)
> * SLEEP and WAKE sets. If AMCs are busy, controller can not enter
> * power collapse, so deny from the last cpu's pm notification.
> *
> + * Context: Must be called with the drv->lock held.
> + *
> * Return:
> * * False - AMCs are idle
> * * True - AMCs are busy
> @@ -754,9 +756,6 @@ static bool rpmh_rsc_ctrlr_is_busy(struct rsc_drv *drv)
> * dedicated TCS for active state use, then re-purposed wake TCSes
> * should be checked for not busy, because we used wake TCSes for
> * active requests in this case.
> - *
> - * Since this is called from the last cpu, need not take drv or tcs
> - * lock before checking tcs_is_free().
> */
> if (!tcs->num_tcs)
> tcs = &drv->tcs[WAKE_TCS];
> @@ -791,36 +790,36 @@ static int rpmh_rsc_cpu_pm_callback(struct notifier_block *nfb,
> {
> struct rsc_drv *drv = container_of(nfb, struct rsc_drv, rsc_pm);
> int ret = NOTIFY_OK;
> -
> - spin_lock(&drv->pm_lock);
> + int cpus_in_pm;
>
> switch (action) {
> case CPU_PM_ENTER:
> - cpumask_set_cpu(smp_processor_id(), &drv->cpus_entered_pm);
> -
> - if (!cpumask_equal(&drv->cpus_entered_pm, cpu_online_mask))
> - goto exit;
> + cpus_in_pm = atomic_inc_return(&drv->cpus_in_pm);
> + if (cpus_in_pm < num_online_cpus())
> + return NOTIFY_OK;
> break;
> case CPU_PM_ENTER_FAILED:
> case CPU_PM_EXIT:
> - cpumask_clear_cpu(smp_processor_id(), &drv->cpus_entered_pm);
> - goto exit;
> - }
> -
> - ret = rpmh_rsc_ctrlr_is_busy(drv);
> - if (ret) {
> - ret = NOTIFY_BAD;
> - goto exit;
> + atomic_dec(&drv->cpus_in_pm);
> + return NOTIFY_OK;
> }
>
> - ret = rpmh_flush(&drv->client);
> - if (ret)
> + /*
> + * It's likely we're on the last CPU. Grab the drv->lock and write
> + * out the sleep/wake commands to RPMH hardware. Grabbing the lock
> + * means that if we race with another CPU coming up we are still
> + * guaranteed to be safe. If another CPU came up just after we checked
> + * and has already started an active transfer then we'll notice we're
> + * busy and abort. If another CPU comes up after we start flushing it
> + * will be blocked from starting an active transfer until we're done
> + * flushing. If another CPU starts an active transfer after we release
> + * the lock we're still OK because we're no longer the last CPU.
> + */
> + spin_lock(&drv->lock);
> + if (rpmh_rsc_ctrlr_is_busy(drv) || !rpmh_flush(&drv->client))
> ret = NOTIFY_BAD;
> - else
> - ret = NOTIFY_OK;
> + spin_unlock(&drv->lock);
>
> -exit:
> - spin_unlock(&drv->pm_lock);
> return ret;
> }
There is a race if we allow other CPUs to exit without pm lock
below scenarios can happen.

1. for the last cpu (CPU-x) CPU_PM_ENTER notification came
2. On CPU-x, rpmh_flush() is in progress with drv->lock held, which is
at the flush_batch() stage
3. Meanwhile any other CPU (CPU-y) woken up and did CPU_PM_EXIT
4. From the woken up cpu, rpmh_invalidate() is invoked.
5. It takes ctrlr->cache_lock and empties the cached batch requests
(making list empty and kfree() the cached requests)
6. The last cpu (CPU-x) which was doing rpmh_flush() suddently gets the
list empty which it was traversing in flush_batch(),
   leading to null pointer derefence.

To Fix above race condtion, ctrlr->cache_lock should be taken inside
rpmh_flush() starting point.
also need to remove lockdep_assert_irqs_disabled() which got insterted,
since IRQ might be enabled by this time if some CPU exits.

Thanks,
Maulik

>
> @@ -964,7 +963,6 @@ static int rpmh_rsc_probe(struct platform_device *pdev)
> solver_config = solver_config >> DRV_HW_SOLVER_SHIFT;
> if (!solver_config) {
> drv->rsc_pm.notifier_call = rpmh_rsc_cpu_pm_callback;
> - spin_lock_init(&drv->pm_lock);
> cpu_pm_register_notifier(&drv->rsc_pm);
> }
>

--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation

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