Re: [PATCH v2 0/2] report stop sched scan when actually done

[Luciano Coelho <coelho@ti.com>]

On Wed, 2011-12-21 at 16:10 +0100, Johannes Berg wrote: 
> On Wed, 2011-12-21 at 12:11 +0200, Eyal Shapira wrote:
> > Changes in mac/cfg80211 to notify userspace
> > of NL80211_CMD_SCHED_SCAN_STOPPED only when the driver
> > actually reported back that it was stopped. Also
> > blocks other attempts until we're really done.
> > 
> > This fixes a scenario where stop sched scan
> > is issued and immediately afterwards a new sched scan
> > is requested (e.g. with other parameters).
> > Current state caused a race where the driver started
> > stopping the sched scan but didn't finish and got
> > another sched scan request which it couldn't handle.
> 
> Luca really needs to take a close look at this, and I think you should
> also take a look to see if this kind of API change can be avoided.

I had discussed this with Eyal briefly and at first it seemed ok.  But
actually I now agree that this is an API change.

Looking closer, I see that in the current documentation, there's nothing
stating that after sending sched_scan_stop the userspace needs to wait
for the stopped event.  It just says that "[this] event is also sent
when the %NL80211_CMD_STOP_SCHED_SCAN command is received [...]".
Applications may assume that after the stop command completes, it is
really stopped, without waiting for the stopped event.

Could the code be changed so that we delay the STOP_SCHED_SCAN command
completion instead? Then userspace can rely on that (as it should
anyway, because the command can fail) instead of having to wait for the
stopped event (which is a change in the API).

-- 
Cheers,
Luca.