Date:   Tue, 19 Mar 2019 17:23:25 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To:     Stanislaw Gruszka <sgruszka@redhat.com>
Cc:     Lorenzo Bianconi <lorenzo@kernel.org>, nbd@nbd.name,
        linux-wireless@vger.kernel.org
Subject: Re: [RFC] mt76: usb: reduce locking in mt76u_tx_tasklet
Message-ID: <20190319162324.GB21821@localhost.localdomain>
References: <cover.1552907276.git.lorenzo@kernel.org>
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 <20190319110708.GA5360@redhat.com>
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 <20190319160426.GA15616@redhat.com>
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> 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
> > > >=20
> > > > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > > > ---
> > > >  drivers/net/wireless/mediatek/mt76/usb.c | 18 +++++++++++-------
> > > >  1 file changed, 11 insertions(+), 7 deletions(-)
> > > >=20
> > > > 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 da=
ta)
> > > >  	int i;
> > > > =20
> > > >  	for (i =3D 0; i < IEEE80211_NUM_ACS; i++) {
> > > > +		u32 n_queued =3D 0, n_sw_queued =3D 0;
> > > > +
> > > >  		sq =3D &dev->q_tx[i];
> > > >  		q =3D sq->q;
> > > > =20
> > > > -		spin_lock_bh(&q->lock);
> > > > -		while (true) {
> > > > +		while (q->queued > n_queued) {
> > > >  			buf =3D &q->entry[q->head].ubuf;
> > > > -			if (!buf->done || !q->queued)
> > > > +			if (!buf->done)
> > > >  				break;
> > >=20
> > > 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.
> >=20
> > Hi Stanislaw,
> >=20
> > I was wondering if this is safe as well, but q->queued is updated holdi=
ng q->lock
> > and I guess it will ensure to not overlap tx and status code path.
>=20
> Overlap will not happen, at worst what can happen is q->queued will be
> smaller on tx_tasklet than on tx_queue_skb.

Yes, that is the point :)

>=20
> > Regarding buf->done, it is already updated without holding the lock in =
mt76u_complete_tx
>=20
> 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 iterati=
on.
> Worse thing would be opposite situation.

Can this really occur? (since queued is update holding the lock)

>=20
> Stanislaw

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