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

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

[Saravana Kannan]

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://lore.kernel.org/lkml/CAA+hA=S4eAreb7vo69LAXSk2t5=DEKNxHaiY1wSpk4xTp9urLg@mail.gmail.com/
Reported-by: Dong Aisheng <dongas86@gmail.com>
Signed-off-by: Saravana Kannan <saravanak@google.com>
---

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


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

[Peng Fan]

> 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

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

[Stephen Boyd]

Quoting Saravana Kannan (2020-09-01 11:44:44)
> 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).

Just curious, why are there sysfs name collisions for device links?
Shouldn't the device link device be named something like "devlink<N>"
with some IDA incrementing N so that collisions can never happen? If
they were always unique then presumably it would be OK to keep using
SRCU?

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