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Subject: Re: [PATCH] mt76: disable softirqs while calling ieee80211_rx_napi
To:     Markus Theil <markus.theil@tu-ilmenau.de>
Cc:     lorenzo.bianconi@redhat.com, sgruszka@redhat.com,
        linux-wireless@vger.kernel.org
References: <20191104134540.19199-1-markus.theil@tu-ilmenau.de>
From:   Felix Fietkau <nbd@nbd.name>
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Date:   Wed, 20 Nov 2019 12:36:55 +0100
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On 2019-11-04 14:45, Markus Theil wrote:
> mac80211 assumes ieee80211_rx_napi to be called with disabled softirqs.
> 
> ieee80211_rx_napi in mac80211.c can be called from aggregation reordering work queue
> or from mt76_rx_poll_complete. mt76_rx_poll_complete does currently not disable softirq
> processing.
> 
> This patch fixes this by disabling softirqs before calling ieee80211_rx_napi.
> It should be no problem to disable them twice, if mt76_aggr_reorder_work calls ieee80211_rx_napi
> and has already called local_bh_disable, as local_bh_disable/local_bh_enable are reentrant.
> 
> I became aware of this issue by the following dmesg output:
>   NOHZ: local_softirq_pending 08
> 
> Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
I believe this fix is incomplete. If we run with softirq enabled at this
point, it also implies that we've taken the RCU lock with softirq
enabled, which we really shouldn't do.
I believe this should be fixed by changing rcu_read_lock/unlock to the
_bh variant in mt76_dma_rx_poll(). I will send a patch for that.

Thanks,

- Felix