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

[Johannes Berg <johannes@sipsolutions.net>]

On Fri, 2015-01-23 at 13:36 +0200, Emmanuel Grumbach wrote:

> > It's not actually just "one packet for each vif/ac" - it's a whole tail
> > of whatever other RCU usages there are. Back when this was discussed,
> > the wizery people measured this to hundreds of ms in many not too
> > uncommon cases.
> 
> That's the part I didn't know. I wasn't aware that this
> synchronize_net() was there and that someone removed it.

This particular one might not have been there - I don't remember. There
were some in this path and we removed quite a few. However, if you have
keys, wpa_s is currently clearing them, which calls it for every single
key. I once had worked on an optimisation here (telling wpa_s not to
clear keys because we handle that when stations are deleted anyway) but
so far that didn't really go anywhere.

> I just wonder how we handle the case where we still have packets in
> the Tx path and we bring everything down. I can check the code, but
> not right now.

Well - I didn't expect you to check anything now :-)

I was more noting this down for 'future work'. It's possible, perhaps,
that we cannot do anything else here, but then we should think about the
other optimisations again...

johannes