Re: [PATCH] mac80211: check whether scan is in progress before queueing scan_work

[Johannes Berg <johannes@sipsolutions.net>]

On Tue, 2010-04-06 at 13:16 +0300, Teemu Paasikivi wrote:
> On Tue, 2010-04-06 at 11:06 +0200, ext Johannes Berg wrote:
> > On Tue, 2010-04-06 at 11:54 +0300, Teemu Paasikivi wrote:
> > > As scan_work is queued from work_work it needs to be checked if scan
> > > has been started during execution of work_work. Otherwise, when hw
> > > scan is used, the stack gets error about hw being busy with ongoing
> > > scan.
> > 
> > Does that mean we ask the driver to scan twice? And your particular
> > driver returns busy?
> > 
> 
> Yes. There seems to be a possibility, that when ieee80211_work_work is
> being executed, __ieee80211_start_scan gets called and it starts a hw
> scan, and also sets local->hw_scan_req etc. (because it looks like
> there's some holes in use of scan_mtx). Result is that work_work queues
> ieee80211_scan_work and when it is executed, as there's already
> hw_scan_req set, it will try to start hw scan again. And as the driver
> used returns busy, scan_work will call ieee80211_scan_completed function
> which leaves the driver (hw more precisely) scanning and the stack
> thinking that it is not anymore scanning.
> 
> Obviously this kind of situation doesn't happen very often in normal
> use, but it can be caused quite easily by associating to access points
> in a loop while running scan in another loop.

Makes some sense. Ignore my other email(s), I looked at this in more
detail now.

It would appear that we need to fix some of the locking here,
potentially simply using a single mutex for both work and scan?

johannes