[PATCH RFC v1 1/1] net: mac80211: fortify the spinlock against deadlock in interrupt

[Mirsad Goran Todorovac]

In the function ieee80211_tx_dequeue() there is a locking sequence:

begin:
	spin_lock(&local->queue_stop_reason_lock);
	q_stopped = local->queue_stop_reasons[q];
	spin_unlock(&local->queue_stop_reason_lock);

However small the chance (increased by ftracetest), an asynchronous
interrupt can occur in between of spin_lock() and spin_unlock(),
and the interrupt routine will attempt to lock the same
&local->queue_stop_reason_lock again.

This is the only remaining spin_lock() on local->queue_stop_reason_lock
that did not disable interrupts and could have possibly caused the deadlock
on the same CPU (core).

This will cause a costly reset of the CPU and wifi device or an
altogether hang in the single CPU and single core scenario.

This is the probable reproduce of the deadlock:

Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:  Possible unsafe locking scenario:
Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:        CPU0
Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:        ----
Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:   lock(&local->queue_stop_reason_lock);
Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:   <Interrupt>
Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:     lock(&local->queue_stop_reason_lock);
Apr 10 00:58:33 marvin-IdeaPad-3-15ITL6 kernel:
                                                 *** DEADLOCK ***

Fixes: 4444bc2116ae
Link: https://lore.kernel.org/all/1f58a0d1-d2b9-d851-73c3-93fcc607501c@xxxxxxxxxxxx/
Cc: Alexander Wetzel <alexander@xxxxxxxxxxxxxx>
Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@xxxxxxxxxxxx>
---
 net/mac80211/tx.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
index 7699fb410670..45cb8e7bcc61 100644
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c
@@ -3781,6 +3781,7 @@ struct sk_buff *ieee80211_tx_dequeue(struct ieee80211_hw *hw,
 	ieee80211_tx_result r;
 	struct ieee80211_vif *vif = txq->vif;
 	int q = vif->hw_queue[txq->ac];
+	unsigned long flags;
 	bool q_stopped;
 
 	WARN_ON_ONCE(softirq_count() == 0);
@@ -3789,9 +3790,9 @@ struct sk_buff *ieee80211_tx_dequeue(struct ieee80211_hw *hw,
 		return NULL;
 
 begin:
-	spin_lock(&local->queue_stop_reason_lock);
+	spin_lock_irqsave(&local->queue_stop_reason_lock, flags);
 	q_stopped = local->queue_stop_reasons[q];
-	spin_unlock(&local->queue_stop_reason_lock);
+	spin_unlock_irqrestore(&local->queue_stop_reason_lock, flags);
 
 	if (unlikely(q_stopped)) {
 		/* mark for waking later */
-- 
2.30.2