Re: [PATCH] ath9k: unlock rcu read when returning early

[Tobias Klausmann]

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@xxxxxxxxxx>
> > ---
> >   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