Re: [PATCH v1] driver core: Fix device_pm_lock() locking for device links

[Dong Aisheng]

Hi Saravana

On Tue, Sep 1, 2020 at 6:10 AM Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
>
> This commit fixes two issues:
>
> 1. The lockdep warning reported by Dong Aisheng <dongas86@xxxxxxxxx> [1].
>
> It is a warning about a cycle (dpm_list_mtx --> kn->active#3 --> fw_lock)
> that was introduced when device-link devices were added to expose device
> link information in sysfs.
>
> The patch that "introduced" this cycle can't be reverted because it's fixes
> a real SRCU issue and also ensures that the device-link device is deleted
> as soon as the device-link is deleted. This is important to avoid sysfs
> name collisions if the device-link is create again immediately (this can
> happen a lot with deferred probing).
>
> 2. device_link_drop_managed() is not grabbing device_pm_lock().
>
> When device_link_del() calls __device_link_del() (device_link_del() ->
> device_link_put_kref() kref_put() -> __device_link_del()) it grabs the
> device_pm_lock().
>
> However, when device_link_drop_managed() calls __device_link_del()
> (device_link_drop_managed() -> kref_put() -> __device_link_del()) it
> doesn't grab device_pm_lock(). There's nothing special about managed
> device-links that remove the need for grabbing device_pm_lock(). So, this
> patch makes sure device_pm_lock() is always held when deleting managed
> links.
>
> And thanks to Stephen Boyd for helping me understand the lockdep splat.
>
> Fixes: 843e600b8a2b ("driver core: Fix sleeping in invalid context during device link deletion")
> Fixes: 515db266a9da ("driver core: Remove device link creation limitation")
> [1] - https://lore.kernel.org/lkml/CAA+hA=S4eAreb7vo69LAXSk2t5=DEKNxHaiY1wSpk4xTp9urLg@xxxxxxxxxxxxxx/
> Reported-by: Dong Aisheng <dongas86@xxxxxxxxx>
> Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx>

Thanks a lot for the quick fix. It worked for me.

Tested-by: Dong Aisheng <aisheng.dong@xxxxxxx>

Regards
Aisheng

> ---
>
> Rafael,
>
> A bigger question I had is why we need to grab device_pm_lock() around
> device_link_del() in the first place. I understand the need to grab it
> during device_link_add() -- it's because we are checking the supplier is
> in the dpm_list and because we are reordering devices on the dpm_list.
>
> But during deletion, we don't need to do either one of those.  So, why
> do we even need to grab the device_pm_lock() in the first place. The
> device_links_write_lock() that we already grab before deleting a device
> link seems like it'd be sufficient. If you agree we don't need to grab
> device_pm_lock() during deletion, then I can change this patch to just
> delete that locking.
>
> -Saravana
>
>  drivers/base/core.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index f6f620aa9408..de1935e21d97 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -766,8 +766,10 @@ static void __device_link_del(struct kref *kref)
>         if (link->flags & DL_FLAG_PM_RUNTIME)
>                 pm_runtime_drop_link(link->consumer);
>
> +       device_pm_lock();
>         list_del_rcu(&link->s_node);
>         list_del_rcu(&link->c_node);
> +       device_pm_unlock();
>         device_unregister(&link->link_dev);
>  }
>  #else /* !CONFIG_SRCU */
> @@ -781,8 +783,10 @@ static void __device_link_del(struct kref *kref)
>         if (link->flags & DL_FLAG_PM_RUNTIME)
>                 pm_runtime_drop_link(link->consumer);
>
> +       device_pm_lock();
>         list_del(&link->s_node);
>         list_del(&link->c_node);
> +       device_pm_unlock();
>         device_unregister(&link->link_dev);
>  }
>  #endif /* !CONFIG_SRCU */
> @@ -807,9 +811,7 @@ static void device_link_put_kref(struct device_link *link)
>  void device_link_del(struct device_link *link)
>  {
>         device_links_write_lock();
> -       device_pm_lock();
>         device_link_put_kref(link);
> -       device_pm_unlock();
>         device_links_write_unlock();
>  }
>  EXPORT_SYMBOL_GPL(device_link_del);
> @@ -830,7 +832,6 @@ void device_link_remove(void *consumer, struct device *supplier)
>                 return;
>
>         device_links_write_lock();
> -       device_pm_lock();
>
>         list_for_each_entry(link, &supplier->links.consumers, s_node) {
>                 if (link->consumer == consumer) {
> @@ -839,7 +840,6 @@ void device_link_remove(void *consumer, struct device *supplier)
>                 }
>         }
>
> -       device_pm_unlock();
>         device_links_write_unlock();
>  }
>  EXPORT_SYMBOL_GPL(device_link_remove);
> --
> 2.28.0.402.g5ffc5be6b7-goog
>