RE: [PATCH v2] driver core: Fix device_pm_lock() locking for device links

From: Peng Fan <peng.fan@nxp.com>
To: Saravana Kannan <saravanak@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>
Cc: dl-linux-imx <linux-imx@nxp.com>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	Aisheng Dong <aisheng.dong@nxp.com>,
	Shawn Guo <shawnguo@kernel.org>,
	Sascha Hauer <kernel@pengutronix.de>,
	"linux@roeck-us.net" <linux@roeck-us.net>,
	"m.szyprowski@samsung.com" <m.szyprowski@samsung.com>,
	"naresh.kamboju@linaro.org" <naresh.kamboju@linaro.org>,
	Stephen Boyd <sboyd@kernel.org>,
	Dong Aisheng <dongas86@gmail.com>,
	"kernel-team@android.com" <kernel-team@android.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH v2] driver core: Fix device_pm_lock() locking for device links
Date: Wed, 2 Sep 2020 01:11:42 +0000	[thread overview]
Message-ID: <DB6PR0402MB27601BDD395CC7F5B2D33C0D882F0@DB6PR0402MB2760.eurprd04.prod.outlook.com> (raw)
In-Reply-To: <20200901184445.1736658-1-saravanak@google.com>

> Subject: [PATCH v2] driver core: Fix device_pm_lock() locking for device links
> 
> This commit fixes two issues:
> 
> 1. The lockdep warning reported by Dong Aisheng <dongas86@gmail.com>
> [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. Inconsistency in grabbing device_pm_lock() during device link deletion
> 
> Some device link deletion code paths grab device_pm_lock(), while others
> don't.  The device_pm_lock() is grabbed during device_link_add() because it
> checks if the supplier is in the dpm_list and also reorders the dpm_list.
> However, when a device link is deleted, it does not do either of those and
> therefore device_pm_lock() is not necessary. Dropping the device_pm_lock()
> in all the device link deletion paths removes the inconsistency in locking.
> 
> Thanks to Stephen Boyd for helping me understand the lockdep splat.
> 
> Fixes: 843e600b8a2b ("driver core: Fix sleeping in invalid context during
> device link deletion") [1] -
> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.ke
> rnel.org%2Flkml%2FCAA%2BhA%3DS4eAreb7vo69LAXSk2t5%3DDEKNxHaiY1
> wSpk4xTp9urLg%40mail.gmail.com%2F&amp;data=02%7C01%7Cpeng.fan%4
> 0nxp.com%7Cc07e23dccfa84d96b17808d84ea71bb9%7C686ea1d3bc2b4c6f
> a92cd99c5c301635%7C0%7C0%7C637345826922594698&amp;sdata=X0Pzb
> ni5QcjxOCWkfR9uvxRcfvpzPQSNMmk%2BJf93dYI%3D&amp;reserved=0
> Reported-by: Dong Aisheng <dongas86@gmail.com>
> Signed-off-by: Saravana Kannan <saravanak@google.com>

Tested-by: Peng Fan <peng.fan@nxp.com>

Thanks,
Peng.

> ---
> 
> Cc'ing everyone from the original thread [1]
> 
> -Saravana
> 
>  drivers/base/core.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c index
> f6f620aa9408..07e5ceb40bb1 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -807,9 +807,7 @@ 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 +828,6 @@ 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 +836,6 @@ 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