On Mon, 2013-02-04 at 18:14 +0100, Christian Lamparter wrote:
> On Monday, February 04, 2013 04:28:28 PM Johannes Berg wrote:
> > On Mon, 2013-02-04 at 16:48 +0530, Amit Shakya wrote:
> > > @@ -2790,7 +2791,20 @@ static void ieee80211_rx_handlers(struct ieee80211_rx_data *rx)
> > >
> > > rx->local->running_rx_handler = true;
> > >
> > > - while ((skb = __skb_dequeue(&rx->local->rx_skb_queue))) {
> > > + skb_queue_walk_safe(&rx->local->rx_skb_queue, skb, tmp) {
> > > + if (!skb)
> > > + break;
> > > + hdr = (struct ieee80211_hdr *) skb->data;
> > > + /*
> > > + * Additional check to ensure that the packets corresponding
> > > + * to same sta entry as in rx->sta are de-queued. The queue
> > > + * can have different interface packets in case of multiple vifs
> > > + */
> > > + if ((rx->sta && hdr) && (ieee80211_is_data(hdr->frame_control))
> > > + && (memcmp(rx->sta->sta.addr, hdr->addr2, ETH_ALEN)))
> > > + continue;
> > > + __skb_unlink(skb, &rx->local->rx_skb_queue);
> >
> > Christian, is there any reason to not just have the queue be on the
> > stack, and use a separate spinlock in the local struct to lock out the
> > unwanted concurrency?
> Let's see.
>
> The original "AMPDU rx reorder timeout timer" had the rx_skb_queue (frames)
> on the stack. But that didn't work because the rx-path isn't thread-safe. This
> issue was addressed by "mac80211: serialize rx path workers" (24a8fda).
It seems this actually caused the problem, because this part:
Only one active rx handler worker [ieee80211_rx_handlers]
is needed. All other threads which have lost the race of
"runnning_rx_handler" can now simply "return", knowing that
the thread who had the "edge" will also take care of their
workload.
forgot to account for the fact that the on-stack versions of "struct
ieee80211_rx_data" can be different. Right?
> Interestingly, the RFC [1] of this patch mentioned the reason why I/we didn't
> go for a rx-path lock:
> " 1. Locking is easy to implement but hard to maintain.
> Furthermore, Johannes worked very hard to get rid
> of as many as possible."
>
> > It seems to me that should work just as well, since there are never frames
> > on the rx_skb_queue for very long, right?
> Yes it should. At least we didn't find anything wrong with it back then.
But ... that doesn't necessarily mean an RX path lock, does it?
I mean, in order to fix the above, we *do* have to make the RX
tasklet/timer wait for each other. So it's not really a big difference
between what we do now and having one of them block, is it? I guess that
they can still do all the local work, and then call the RX handlers with
the lock held? Hmm. That does kinda mean an RX path lock :-)
I guess it's the only way I see, since we can't really disable RX from
drivers when the timer starts running.
johannes
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