linux-kernel - [PATCH v2] driver core: Fix device_pm

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Message-Id: <20200901184445.1736658-1-saravanak@google.com>
Date:   Tue,  1 Sep 2020 11:44:44 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Saravana Kannan <saravanak@...gle.com>
Cc:     linux-imx@....com, linux-arm-kernel@...ts.infradead.org,
        Dong Aisheng <aisheng.dong@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <kernel@...gutronix.de>, linux@...ck-us.net,
        m.szyprowski@...sung.com, naresh.kamboju@...aro.org,
        Peng Fan <peng.fan@....com>, Stephen Boyd <sboyd@...nel.org>,
        Dong Aisheng <dongas86@...il.com>, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org
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@...il.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@...il.com>
Signed-off-by: Saravana Kannan <saravanak@...gle.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