Re: [PATCH v2] mac80211: fix iflist_mtx/mtx locking in radar detection

[Johannes Berg <johannes@sipsolutions.net>]

On Wed, 2013-12-18 at 20:45 +0100, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@intel.com>
> 
> The scan code creates an iflist_mtx -> mtx locking dependency,
> and a few other places, notably radar detection, were creating
> the opposite dependency, causing lockdep to complain. As scan
> and radar detection are mutually exclusive, the deadlock can't
> really happen in practice, but it's still bad form.
> 
> A similar issue exists in the monitor mode code, but this is
> only used by channel-context drivers right now and those have
> to have hardware scan, so that also can't happen.
> 
> Still, fix these issues by making some of the channel context
> code require the mtx to be held rather than acquiring it, thus
> allowing the monitor/radar callers to keep the iflist_mtx->mtx
> lock ordering.
> 
> While at it, also fix access to the local->scanning variable
> in the radar code, and document that radar_detect_enabled is
> now properly protected by the mtx.
> 
> All this would now introduce an ABBA deadlock between the DFS
> work cancelling and local->mtx, so change the locking there a
> bit to not need to use cancel_delayed_work_sync() but be able
> to just use cancel_delayed_work(). The work is also safely
> stopped/removed when the interface is stopped, so no extra
> changes are needed.

Applied.

johannes