Re: [RFC] mt76: usb: reduce locking in mt76u_tx_tasklet

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Tue, Mar 19, 2019 at 01:58:13PM +0100, Lorenzo Bianconi wrote:
> > On Mon, Mar 18, 2019 at 12:09:32PM +0100, Lorenzo Bianconi wrote:
> > > Similar to pci counterpart, reduce locking in mt76u_tx_tasklet since
> > > q->head is managed just in mt76u_tx_tasklet and q->queued is updated
> > > holding q->lock
> > > 
> > > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > > ---
> > >  drivers/net/wireless/mediatek/mt76/usb.c | 18 +++++++++++-------
> > >  1 file changed, 11 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/drivers/net/wireless/mediatek/mt76/usb.c b/drivers/net/wireless/mediatek/mt76/usb.c
> > > index ac03acdae279..8cd70c32d77a 100644
> > > --- a/drivers/net/wireless/mediatek/mt76/usb.c
> > > +++ b/drivers/net/wireless/mediatek/mt76/usb.c
> > > @@ -634,29 +634,33 @@ static void mt76u_tx_tasklet(unsigned long data)
> > >  	int i;
> > >  
> > >  	for (i = 0; i < IEEE80211_NUM_ACS; i++) {
> > > +		u32 n_queued = 0, n_sw_queued = 0;
> > > +
> > >  		sq = &dev->q_tx[i];
> > >  		q = sq->q;
> > >  
> > > -		spin_lock_bh(&q->lock);
> > > -		while (true) {
> > > +		while (q->queued > n_queued) {
> > >  			buf = &q->entry[q->head].ubuf;
> > > -			if (!buf->done || !q->queued)
> > > +			if (!buf->done)
> > >  				break;
> > 
> > I'm still thinking if this is safe or not. Is somewhat tricky to
> > read variable outside the lock because in such case there is no time
> > guarantee when variable written on one CPU gets updated value on
> > different CPU. And for USB is not only q->queued but also buf->done.
> 
> Hi Stanislaw,
> 
> I was wondering if this is safe as well, but q->queued is updated holding q->lock
> and I guess it will ensure to not overlap tx and status code path.

Overlap will not happen, at worst what can happen is q->queued will be
smaller on tx_tasklet than on tx_queue_skb.

> Regarding buf->done, it is already updated without holding the lock in mt76u_complete_tx

That's actually a bug, but it's not important, if tx_tasklet will not
see updated buf->done <- true value by mt76u_complete_tx on different
cpu, it will not complete skb. It will be done on next tx_tasklet iteration.
Worse thing would be opposite situation.

Stanislaw