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

[Lorenzo Bianconi <lorenzo@kernel.org>]

[-- Attachment #1: Type: text/plain, Size: 3196 bytes --]

> On Tue, Mar 19, 2019 at 05:23:25PM +0100, Lorenzo Bianconi wrote:
> > > 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.
> > 
> > Yes, that is the point :)
> > 
> > > 
> > > > 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.
> > 
> > Can this really occur?
> I was thinking about that and yes it can occur. If q->queued and
> buf->done writes/read will be reordered by CPUs. To prevent that you 
> will need to use smp_wmb/smp_rmb pair, but it's just simpler and more
> convenient to use lock.

good point, I will go through it.

Regards,
Lorenzo

> 
> > (since queued is update holding the lock)
> Holding the lock on one thread without holding it on concurrent thread
> is irrelevant, it's the same as not holding any lock at all.
> 
> Stanislaw

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]