[RFC] PM / QoS: Decouple request alloc from dev_pm_qos_mtx (alternative solution)

[Rafael J. Wysocki]

On Fri, Aug 4, 2023 at 11:41 PM Rob Clark <robdclark at gmail.com> wrote:
>
> From: Rob Clark <robdclark at chromium.org>
>
> Similar to the previous patch, move the allocation out from under
> dev_pm_qos_mtx, by speculatively doing the allocation and handle
> any race after acquiring dev_pm_qos_mtx by freeing the redundant
> allocation.
>
> Suggested-by: Rafael J. Wysocki <rafael at kernel.org>
> Signed-off-by: Rob Clark <robdclark at chromium.org>
> ---
> This is an alternative to https://patchwork.freedesktop.org/patch/551417/?series=115028&rev=4
>
> So, this does _slightly_ change error paths, for ex
> dev_pm_qos_update_user_latency_tolerance() will now allocate
> dev->power.qos in some error cases.  But this seems harmless?

It is harmless AFAICS.

> A slightly more complicated version of this could conserve the
> previous error path behavior, but I figured I'd try the simpler
> thing first.

Good choice!

>  drivers/base/power/qos.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/base/power/qos.c b/drivers/base/power/qos.c
> index 1b73a704aac1..c7ba85e89c42 100644
> --- a/drivers/base/power/qos.c
> +++ b/drivers/base/power/qos.c
> @@ -920,8 +920,12 @@ s32 dev_pm_qos_get_user_latency_tolerance(struct device *dev)
>  int dev_pm_qos_update_user_latency_tolerance(struct device *dev, s32 val)
>  {
>         struct dev_pm_qos *qos = dev_pm_qos_constraints_allocate();
> +       struct dev_pm_qos_request *req = NULL;
>         int ret = 0;
>
> +       if (!dev->power.qos->latency_tolerance_req)
> +               req = kzalloc(sizeof(*req), GFP_KERNEL);
> +
>         mutex_lock(&dev_pm_qos_mtx);
>
>         dev_pm_qos_constraints_set(dev, qos);
> @@ -935,8 +939,6 @@ int dev_pm_qos_update_user_latency_tolerance(struct device *dev, s32 val)
>                 goto out;
>
>         if (!dev->power.qos->latency_tolerance_req) {
> -               struct dev_pm_qos_request *req;
> -
>                 if (val < 0) {
>                         if (val == PM_QOS_LATENCY_TOLERANCE_NO_CONSTRAINT)
>                                 ret = 0;
> @@ -944,17 +946,15 @@ int dev_pm_qos_update_user_latency_tolerance(struct device *dev, s32 val)
>                                 ret = -EINVAL;
>                         goto out;
>                 }
> -               req = kzalloc(sizeof(*req), GFP_KERNEL);
>                 if (!req) {
>                         ret = -ENOMEM;
>                         goto out;
>                 }
>                 ret = __dev_pm_qos_add_request(dev, req, DEV_PM_QOS_LATENCY_TOLERANCE, val);
> -               if (ret < 0) {
> -                       kfree(req);
> +               if (ret < 0)
>                         goto out;
> -               }
>                 dev->power.qos->latency_tolerance_req = req;
> +               req = NULL;
>         } else {
>                 if (val < 0) {
>                         __dev_pm_qos_drop_user_request(dev, DEV_PM_QOS_LATENCY_TOLERANCE);
> @@ -966,6 +966,7 @@ int dev_pm_qos_update_user_latency_tolerance(struct device *dev, s32 val)
>
>   out:
>         mutex_unlock(&dev_pm_qos_mtx);
> +       kfree(req);
>         return ret;
>  }
>  EXPORT_SYMBOL_GPL(dev_pm_qos_update_user_latency_tolerance);
> --

Yes, something like this, please!