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Subject: Re: [PATCH] ath9k: unlock rcu read when returning early
To: Felix Fietkau <nbd@nbd.name>, kvalo@codeaurora.org, helgaas@kernel.org,
        linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        marc.zyngier@arm.com, Janusz.Dziedzic@tieto.com,
        rmanohar@qti.qualcomm.com, ath9k-devel@qca.qualcomm.com,
        linux-wireless@vger.kernel.org, rmanohar@qca.qualcomm.com,
        bharat.kumar.gogada@xilinx.com
References: <1cFMHb-00BdWd-Ks>
 <20161212185001.3857-1-tobias.johannes.klausmann@mni.thm.de>
 <e15c5efc-d548-8b32-ca86-5ec26506ff7c@nbd.name>
From: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Message-ID: <7b4b7748-06d6-92d4-228c-e7ebf00f8699@mni.thm.de>
Date: Tue, 13 Dec 2016 14:41:12 +0100
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On 13.12.2016 11:41, Felix Fietkau wrote:
> On 2016-12-12 19:50, Tobias Klausmann wrote:
>> 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@mni.thm.de>
>> ---
>>   drivers/net/wireless/ath/ath9k/xmit.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/net/wireless/ath/ath9k/xmit.c b/drivers/net/wireless/ath/ath9k/xmit.c
>> index 52bfbb988611..857d5ae09a1d 100644
>> --- a/drivers/net/wireless/ath/ath9k/xmit.c
>> +++ b/drivers/net/wireless/ath/ath9k/xmit.c
>> @@ -2787,6 +2787,7 @@ void ath_tx_edma_tasklet(struct ath_softc *sc)
>>   		fifo_list = &txq->txq_fifo[txq->txq_tailidx];
>>   		if (list_empty(fifo_list)) {
>>   			ath_txq_unlock(sc, txq);
>> +			rcu_read_unlock();
> Technically this is fine as well, but I'd prefer a fix where you replace
> the 'return' with 'break', thus avoiding the duplication of
> rcu_read_unlock()

Actually if you want to avoid it, maybe skipping over the rest is better 
(as originally intended):

...

ath_txq_unlock(sc, txq);


goto unlock;
}
...

unlock:
rcu_read_unlock();

Thanks,
Tobias
>
> Thanks,
>
> - Felix
>