linux-kernel - [PATCH] [RFC] EEE PC hangs when booting off battery

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Date:	Tue, 28 Apr 2009 10:19:10 +0100
From:	Alan Jenkins <alan-jenkins@...fmail.co.uk>
To:	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
CC:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: [PATCH] [RFC] EEE PC hangs when booting off battery

I found a regression where my EEE hangs at boot time, if the battery is
present.

I'm confident it's a regression because it disappears if I revert
Arjan's asynchronous battery initialisation.  However, the evidence
points to a deadlock in the wireless stack which has simply been
uncovered by timing changes.

If I leave the system long enough, I get a series of hung task
warnings.  They suggest the following deadlock:

- ieee80211_wep_init(), which is called with rtnl_lock() held, is
blocked in request_module() [waiting for modprobe to load a crypto module].
- modprobe is blocked in a call to flush_workqueue(), caused by closing
a TTY.
- worker_thread is blocked because the workqueue item linkwatch_event()
is blocked on rtnl_lock.


I've hacked up a test patch to move wep_init() outside of rtnl_lock, and
it solved the problem.  My one caveat is that it would probably be
cleaner to move it after rtnl_unlock(), instead of before rtnl_lock(). 
I just wasn't 100% sure if that would be safe.  Here's the patch:

---8<---

diff --git a/net/mac80211/main.c b/net/mac80211/main.c
index fbcbed6..fffa7f9 100644
--- a/net/mac80211/main.c
+++ b/net/mac80211/main.c
@@ -909,6 +909,13 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 	if (result < 0)
 		goto fail_sta_info;
 
+	result = ieee80211_wep_init(local);
+	if (result < 0) {
+		printk(KERN_DEBUG "%s: Failed to initialize wep: %d\n",
+		       wiphy_name(local->hw.wiphy), result);
+		goto fail_wep;
+	}
+
 	rtnl_lock();
 	result = dev_alloc_name(local->mdev, local->mdev->name);
 	if (result < 0)
@@ -930,14 +937,6 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 		goto fail_rate;
 	}
 
-	result = ieee80211_wep_init(local);
-
-	if (result < 0) {
-		printk(KERN_DEBUG "%s: Failed to initialize wep: %d\n",
-		       wiphy_name(local->hw.wiphy), result);
-		goto fail_wep;
-	}
-
 	/* add one default STA interface if supported */
 	if (local->hw.wiphy->interface_modes & BIT(NL80211_IFTYPE_STATION)) {
 		result = ieee80211_if_add(local, "wlan%d", NULL,
@@ -967,13 +966,12 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 
 	return 0;
 
-fail_wep:
-	rate_control_deinitialize(local);
 fail_rate:
 	unregister_netdevice(local->mdev);
 	local->mdev = NULL;
 fail_dev:
 	rtnl_unlock();
+fail_wep:
 	sta_info_stop(local);
 fail_sta_info:
 	debugfs_hw_del(local);


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