[RFC PATCH v4] Use kernfs_break_active_protection() for device online store callbacks

[Li Zhong <zhong@linux.vnet.ibm.com>]

This patch tries to solve the device hot remove locking issues in a
different way from commit 5e33bc41, as kernfs already has a mechanism 
to break active protection. 

The problem here is the order of s_active, and series of hotplug related
lock. 

This patch takes s_active out of the lock dependency graph, so it won't
dead lock with any hotplug realted locks.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
---
 drivers/base/core.c   | 37 ++++++++++++++++++++++++++++++++++---
 drivers/base/memory.c | 18 +++++++++++++++---
 2 files changed, 49 insertions(+), 6 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 0dd6528..1b96cb9 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -439,17 +439,48 @@ static ssize_t online_store(struct device *dev, struct device_attribute *attr,
 {
 	bool val;
 	int ret;
+	struct kernfs_node *kn;
 
 	ret = strtobool(buf, &val);
 	if (ret < 0)
 		return ret;
 
-	ret = lock_device_hotplug_sysfs();
-	if (ret)
-		return ret;
+	kn = kernfs_find_and_get(dev->kobj.sd, attr->attr.name);
+	if (WARN_ON_ONCE(!kn))
+		return -ENODEV;
+
+	/*
+	 * Break active protection here to avoid deadlocks with device
+	 * removing process, which tries to remove sysfs entries including this
+	 * "online" attribute while holding some hotplug related locks.
+	 *
+	 * @dev needs to be protected here, or it could go away any time after
+	 * dropping active protection. But it is still unreasonable/unsafe to
+	 * online/offline a device after it being removed. Fortunately, there
+	 * are some checks in online/offline knobs. Like cpu, it checks cpu
+	 * present/online mask before doing the real work.
+	 */
+
+	get_device(dev);
+	kernfs_break_active_protection(kn);
+
+	lock_device_hotplug();
+
+	/*
+	 * If we assume device_hotplug_lock must be acquired before removing
+	 * device, we may try to find a way to check whether the device has
+	 * been removed here, so we don't call device_{on|off}line against
+	 * removed device.
+	 */
 
 	ret = val ? device_online(dev) : device_offline(dev);
 	unlock_device_hotplug();
+
+	kernfs_unbreak_active_protection(kn);
+	put_device(dev);
+
+	kernfs_put(kn);
+
 	return ret < 0 ? ret : count;
 }
 static DEVICE_ATTR_RW(online);
diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index bece691..0d2f3a5 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -320,10 +320,17 @@ store_mem_state(struct device *dev,
 {
 	struct memory_block *mem = to_memory_block(dev);
 	int ret, online_type;
+	struct kernfs_node *kn;
 
-	ret = lock_device_hotplug_sysfs();
-	if (ret)
-		return ret;
+	kn = kernfs_find_and_get(dev->kobj.sd, attr->attr.name);
+	if (WARN_ON_ONCE(!kn))
+		return -ENODEV;
+
+	/* refer to comments in online_store() for more information */
+	get_device(dev);
+	kernfs_break_active_protection(kn);
+
+	lock_device_hotplug();
 
 	if (!strncmp(buf, "online_kernel", min_t(int, count, 13)))
 		online_type = ONLINE_KERNEL;
@@ -362,6 +369,11 @@ store_mem_state(struct device *dev,
 err:
 	unlock_device_hotplug();
 
+	kernfs_unbreak_active_protection(kn);
+	put_device(dev);
+
+	kernfs_put(kn);
+
 	if (ret)
 		return ret;
 	return count;