[source] ath9k: add a RCU related bugfix

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/4872c36c55f9423ac3a5859fd4eb42a23545632b

commit 4872c36c55f9423ac3a5859fd4eb42a23545632b
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Tue Dec 13 12:32:13 2016 +0100

    ath9k: add a RCU related bugfix
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 ...th9k-unlock-rcu-read-when-returning-early.patch | 65 ++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/package/kernel/mac80211/patches/329-ath9k-unlock-rcu-read-when-returning-early.patch b/package/kernel/mac80211/patches/329-ath9k-unlock-rcu-read-when-returning-early.patch
new file mode 100644
index 0000000..b8ba355
--- /dev/null
+++ b/package/kernel/mac80211/patches/329-ath9k-unlock-rcu-read-when-returning-early.patch
@@ -0,0 +1,65 @@
+From: Tobias Klausmann <tobias.johannes.klausmann at mni.thm.de>
+Date: Mon, 12 Dec 2016 19:50:01 +0100
+Subject: [PATCH] ath9k: unlock rcu read when returning early
+
+Starting with ath9k: use ieee80211_tx_status_noskb where possible
+[d94a461d7a7df68991fb9663531173f60ef89c68] the driver uses rcu_read_lock() &&
+rcu_read_unlock() yet on returning early in ath_tx_edma_tasklet() the unlock is
+missing leading to stalls and suspicious RCU usage:
+
+ ===============================
+ [ INFO: suspicious RCU usage. ]
+ 4.9.0-rc8 #11 Not tainted
+ -------------------------------
+ kernel/rcu/tree.c:705 Illegal idle entry in RCU read-side critical section.!
+
+ other info that might help us debug this:
+
+ RCU used illegally from idle CPU!
+ rcu_scheduler_active = 1, debug_locks = 0
+ RCU used illegally from extended quiescent state!
+ 1 lock held by swapper/7/0:
+ #0:
+  (
+ rcu_read_lock
+ ){......}
+ , at:
+ [<ffffffffa06ed110>] ath_tx_edma_tasklet+0x0/0x450 [ath9k]
+
+ stack backtrace:
+ CPU: 7 PID: 0 Comm: swapper/7 Not tainted 4.9.0-rc8 #11
+ Hardware name: Acer Aspire V3-571G/VA50_HC_CR, BIOS V2.21 12/16/2013
+  ffff88025efc3f38 ffffffff8132b1e5 ffff88017ede4540 0000000000000001
+  ffff88025efc3f68 ffffffff810a25f7 ffff88025efcee60 ffff88017edebdd8
+  ffff88025eeb5400 0000000000000091 ffff88025efc3f88 ffffffff810c3cd4
+ Call Trace:
+  <IRQ>
+  [<ffffffff8132b1e5>] dump_stack+0x68/0x93
+  [<ffffffff810a25f7>] lockdep_rcu_suspicious+0xd7/0x110
+  [<ffffffff810c3cd4>] rcu_eqs_enter_common.constprop.85+0x154/0x200
+  [<ffffffff810c5a54>] rcu_irq_exit+0x44/0xa0
+  [<ffffffff81058631>] irq_exit+0x61/0xd0
+  [<ffffffff81018d25>] do_IRQ+0x65/0x110
+  [<ffffffff81672189>] common_interrupt+0x89/0x89
+  <EOI>
+  [<ffffffff814ffe11>] ? cpuidle_enter_state+0x151/0x200
+  [<ffffffff814ffee2>] cpuidle_enter+0x12/0x20
+  [<ffffffff8109a6ae>] call_cpuidle+0x1e/0x40
+  [<ffffffff8109a8f6>] cpu_startup_entry+0x146/0x220
+  [<ffffffff810336f8>] start_secondary+0x148/0x170
+
+Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann at mni.thm.de>
+Signed-off-by: Felix Fietkau <nbd at nbd.name>
+---
+
+--- a/drivers/net/wireless/ath/ath9k/xmit.c
++++ b/drivers/net/wireless/ath/ath9k/xmit.c
+@@ -2757,7 +2757,7 @@ void ath_tx_edma_tasklet(struct ath_soft
+ 		fifo_list = &txq->txq_fifo[txq->txq_tailidx];
+ 		if (list_empty(fifo_list)) {
+ 			ath_txq_unlock(sc, txq);
+-			return;
++			break;
+ 		}
+ 
+ 		bf = list_first_entry(fifo_list, struct ath_buf, list);