Date:   Tue, 19 Mar 2019 13:58:13 +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: <20190319125812.GA21821@localhost.localdomain>
References: <cover.1552907276.git.lorenzo@kernel.org>
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 <20190319110708.GA5360@redhat.com>
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> 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/wir=
eless/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;
> > =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.

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.
Regarding buf->done, it is already updated without holding the lock in mt76=
u_complete_tx

Regards,
Lorenzo

> =20
> Stanislaw
>=20

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