Re: [PATCH] mac80211: keep BHs disabled while calling drv_tx_wake_queue()

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Johannes Berg <johannes@sipsolutions.net> writes:

> On Tue, 2019-10-01 at 12:53 +0200, Toke Høiland-Jørgensen wrote:
>> 
>> > -			spin_unlock_bh(&fq->lock);
>> > +			spin_unlock(&fq->lock);
>> >  			drv_wake_tx_queue(local, txqi);
>> > -			spin_lock_bh(&fq->lock);
>> > +			spin_lock(&fq->lock);
>> 
>> Okay, so this will mean that the drv_wake_tx_queue() entry point will be
>> called with bhs disabled.
>
> Right.
>
>>  But there are lots of uses of
>> spin_{,un}lock_bh() in tx.c:
>
> [snip]
>
>> so won't that mean that the driver still gets bhs re-enabled after (for
>> instance) the first call to ieee80211_tx_dequeue()?
>
> No, local_bh_disable()/local_bh_enable() is re-entrant and nests fine.

Ah, right, gotcha. Hmm, in that case, would it be more clear to just add
an outer local_bh_disable()/local_bh_enable() to
___ieee80211_wake_txqs(). With this patch we're relying on the
mismatched use of _bh/non-_bh variants of the locking to ensure the bhs
stay off. Isn't that prone to breaking in the future?

Oh, and also, with just this patch, the additional drv_wake_tx_queue()
call for the vif TXQ at the bottom of __ieee80211_wake_txqs() will still
happen without bhs disabled, won't it?

-Toke