On 2019-03-21 10:02, Lorenzo Bianconi wrote: >> 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 <[email protected]> >> > > > > > --- >> > > > > > 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. Another simple solution would be to set buf->done = false in mt76u_tx_tasklet after tx_complete_skb instead of doing it at enqueue time.
- Felix
