On Tue, 2011-01-11 at 23:11 -0600, Larry Finger wrote:
> My system has logged the following locking problem:
>
> ==================================================================
> [ INFO: inconsistent lock state ]
> 2.6.37-Linus-03737-g0c21e3a-dirty #251
> ---------------------------------
> inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.
> takes:
> (&(&list->lock)->rlock#5){?.-...}, at: skb_queue_tail+0x26/0x60
> {HARDIRQ-ON-W} state was registered at:
> __lock_acquire+0xb25/0x1cc0
> lock_acquire+0x93/0x130
> _raw_spin_lock+0x2c/0x40
> ieee80211_rx_handlers+0x27/0x1c80 [mac80211]
> ieee80211_prepare_and_rx_handle+0x238/0x900 [mac80211]
> ieee80211_rx+0x31a/0x940 [mac80211]
> ieee80211_tasklet_handler+0xc1/0xd0 [mac80211]
> tasklet_action+0x73/0x120
> __do_softirq+0xce/0x200
>
> ==================================================================
>
> The reason is that ieee80211_rx_handlers() locks rx->local->rx_skb_queue.lock
> using spin_lock(), but skb_queue_tail() locks the same entity with
> spin_lock_irqsave().
>
> Signed-off-by: Larry Finger <Larry.Finger@xxxxxxxxxxxx>
> ---
>
> Johannes,
>
> I think this is correct. At least the lockdep warning goes away on my
> machine.
I have to apologize -- I've sorta pushed off looking at this (my excuse
is some important iwlwifi bugs, but ...).
If I look at your original trace again, I see:
[ 25.660384] inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.
This is a tad confusing, because it then goes to print out something
about ieee80211_rx_handlers(), which only acquires the
local->rx_skb_queue.lock; looking further, however, the current stack
trace is ieee80211_tx_status_irqsafe() which uses only
local->skb_queue[_unreliable]. I think Stanislaw was right on (but why
didn't he offer a fix? :-) ).
While your fix certainly isn't incorrect, I believe it to be unnecessary
to disable IRQs here. The lock can only be taken with BHs disabled, but
this is in a BH or running with BHs disabled. Of course, the invocations
of skb_queue_tail() will still do IRQ locking, but I'm willing to pay
that price, for now, until somebody invents skb_queue_tail_bh() :-)
I believe the patch below should address the lockdep warning without the
IRQ disabling.
johannes
--- wireless-testing.orig/net/mac80211/main.c 2011-01-12 09:58:07.000000000 +0100
+++ wireless-testing/net/mac80211/main.c 2011-01-12 10:02:03.000000000 +0100
@@ -39,6 +39,8 @@ module_param(ieee80211_disable_40mhz_24g
MODULE_PARM_DESC(ieee80211_disable_40mhz_24ghz,
"Disable 40MHz support in the 2.4GHz band");
+static struct lock_class_key ieee80211_rx_skb_queue_class;
+
void ieee80211_configure_filter(struct ieee80211_local *local)
{
u64 mc;
@@ -569,7 +571,15 @@ struct ieee80211_hw *ieee80211_alloc_hw(
spin_lock_init(&local->filter_lock);
spin_lock_init(&local->queue_stop_reason_lock);
- skb_queue_head_init(&local->rx_skb_queue);
+ /*
+ * The rx_skb_queue is only accessed from tasklets,
+ * but other SKB queues are used from within IRQ
+ * context. Therefore, this one needs a different
+ * locking class so our direct, non-irq-safe use of
+ * the queue's lock doesn't throw lockdep warnings.
+ */
+ skb_queue_head_init_class(&local->rx_skb_queue,
+ &ieee80211_rx_skb_queue_class);
INIT_DELAYED_WORK(&local->scan_work, ieee80211_scan_work);
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