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

[Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>]

On Tue, 2010-04-06 at 12:29 +0200, ext Johannes Berg wrote:
> On Tue, 2010-04-06 at 12:27 +0200, Johannes Berg wrote:
> > 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?
> 
> I'm mostly worried about deadlocks between work_mtx and scan_mtx really,
> when we acquire them both like your patch.
> 

I sent a new version of the patch to the linux-wireless. Now both
mutexes are not locked at the same time.


Br,

Teemu