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Subject: Re: [PATCH] ath9k: unlock rcu read when returning early
To: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>,
        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>
From: Felix Fietkau <nbd@nbd.name>
Message-ID: <e15c5efc-d548-8b32-ca86-5ec26506ff7c@nbd.name>
Date: Tue, 13 Dec 2016 11:41:07 +0100
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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()

Thanks,

- Felix