linux-kernel - Re: [PATCH] USB: serial: fix sysfs-attribute removal deadlock

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Message-ID: <1398328155.2805.100.camel@ThinkPad-T5421.cn.ibm.com>
Date:	Thu, 24 Apr 2014 16:29:15 +0800
From:	Li Zhong <zhong@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Johan Hovold <jhovold@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alan Stern <stern@...land.harvard.edu>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	rafael.j.wysocki@...el.com
Subject: Re: [PATCH] USB: serial: fix sysfs-attribute removal deadlock

On Wed, 2014-04-23 at 10:19 -0400, Tejun Heo wrote:
> cc'ing Li Zhong who's working on a simliar issue in the following
> thread and quoting whole body.
> 
>   http://thread.gmane.org/gmane.linux.kernel/1680706
> 
> Li, this is another variation of the same problem.  Maybe this can be
> covered by your work too?

It seems to me that it is about write something to driver attribute, and
driver unloading. If so, maybe it's not easy to reuse the help functions
created for device attribute, and device removing.

But I guess the idea to break the active protection could still be
applied here:

Maybe we could try_module_get() here (like the other option suggested by
Johan?), and break active protection if we could get the module,
something like below? 

================
diff --git a/drivers/base/bus.c b/drivers/base/bus.c
index 83e910a..6ce27e0 100644
--- a/drivers/base/bus.c
+++ b/drivers/base/bus.c
@@ -69,9 +69,24 @@ static ssize_t drv_attr_store(struct kobject *kobj, struct attribute *attr,
 	struct driver_attribute *drv_attr = to_drv_attr(attr);
 	struct driver_private *drv_priv = to_driver(kobj);
 	ssize_t ret = -EIO;
+	struct kernfs_node *kn;
+
+	if (!try_module_get(drv_priv->driver->owner))
+		return ret;
+
+	kn = kernfs_find_and_get(kobj->sd, attr->name);
+	if (WARN_ON_ONCE(!kn))
+		return ret;
+
+	kernfs_break_active_protection(kn);
 
 	if (drv_attr->store)
 		ret = drv_attr->store(drv_priv->driver, buf, count);
+
+	kernfs_unbreak_active_protection(kn);
+	kernfs_put(kn);
+
+	module_put(drv_priv->driver->owner);
 	return ret;
 }
 


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