Re: [PATCH] mac80211: synchronize_net() before flushing the queues

[Johannes Berg <johannes@sipsolutions.net>]

On Thu, 2015-01-22 at 23:30 +0200, Emmanuel Grumbach wrote:
> When mac80211 disconnects, it drops all the packets on the
> queues. This happens after the net stack has been notified
> that we have no link anymore (netif_carrier_off).
> netif_carrier_off ensures that no new packets are sent to
> xmit() callback, but we might have older packets in the
> middle of the Tx path. These packets will land in the
> driver's queues after the latter have been flushed.
> Synchronize_net() between netif_carrier_off and drv_flush()
> will fix this.
> 
> Note that we can't call synchronize_net inside
> ieee80211_flush_queues since there are flows that call
> ieee80211_flush_queues and don't need synchronize_net()
> which is an expensive operation.

I'm -- with a heavy heart -- applying this for now. TI/wizery spent a
lot of time optimising the time it takes to roam, and this single line
will introduce a potential for hundreds of ms of latency into the
roaming flow.

You (we) really need to think about how to avoid that.

johannes