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Subject: Re: [PATCH] mt76: use rcu_read_lock_bh in mt76_dma_rx_poll
To:     Felix Fietkau <nbd@nbd.name>, linux-wireless@vger.kernel.org
References: <20191120114647.88967-1-nbd@nbd.name>
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From:   Markus Theil <markus.theil@tu-ilmenau.de>
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Message-ID: <92a53651-d309-4dfe-b14f-1d6aa63cafe5@tu-ilmenau.de>
Date:   Wed, 20 Nov 2019 13:15:39 +0100
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Sender: linux-wireless-owner@vger.kernel.org
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On 11/20/19 1:03 PM, Felix Fietkau wrote:
> On 2019-11-20 12:59, Markus Theil wrote:
>> On 11/20/19 12:46 PM, Felix Fietkau wrote:
>>> Fixes potential RCU issues and avoids calling ieee80211_rx_napi with softirq
>>> enabled.
>>>
>>> Reported-by: Markus Theil <markus.theil@tu-ilmenau.de>
>>> Signed-off-by: Felix Fietkau <nbd@nbd.name>
>>> ---
>>>  drivers/net/wireless/mediatek/mt76/dma.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/net/wireless/mediatek/mt76/dma.c b/drivers/net/wireless/mediatek/mt76/dma.c
>>> index 6173c80189ba..ed0ee2d4a452 100644
>>> --- a/drivers/net/wireless/mediatek/mt76/dma.c
>>> +++ b/drivers/net/wireless/mediatek/mt76/dma.c
>>> @@ -528,7 +528,7 @@ mt76_dma_rx_poll(struct napi_struct *napi, int budget)
>>>  	dev = container_of(napi->dev, struct mt76_dev, napi_dev);
>>>  	qid = napi - dev->napi;
>>>  
>>> -	rcu_read_lock();
>>> +	rcu_read_lock_bh();
>>>  
>>>  	do {
>>>  		cur = mt76_dma_rx_process(dev, &dev->q_rx[qid], budget - done);
>>> @@ -536,7 +536,7 @@ mt76_dma_rx_poll(struct napi_struct *napi, int budget)
>>>  		done += cur;
>>>  	} while (cur && done < budget);
>>>  
>>> -	rcu_read_unlock();
>>> +	rcu_read_unlock_bh();
>>>  
>>>  	if (done < budget && napi_complete(napi))
>>>  		dev->drv->rx_poll_complete(dev, qid);
>> This patch is incomplete. The same problem exists for usb. I'll send a fix for that.
> How so? On usb, it is called from a tasklet already.
>
> - Felix

I've noticed the bug I reported on USB devices. I've only seen that mt76_rx_aggr_reorder_work also accesses mt76_rx_complete.
Maybe it only worked for me by accident. Of course, the tasklet should not run in parallel with multiple instances.

Markus