mac80211: cancel restart_work in ieee80211_unregister_hw

Unlike most other workqueue-tasks, the restart_work is not scheduled onto mac80211's private per-interface workqueue, but onto one of the system-wide workqueues.

Therefore the mac80211-stack has to cancel any pending restarts, before destroying the shared device context and handing back the memory. Otherwise - under very unlucky circumstances - there could be a stale work- item left, because some other kernel component might have delayed the execution of ieee80211_restart_work for too long.

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Sure, normally a device doesn't simply disappear, but you know: restarts might take some time (fw upload, phy/mac/crypto reinit, etc.) to finish and some impatient user might want to speed things up by _simply_ replugging the USB device...

But don't ask me what this "elusive" kernel component could be, I stumbled upon this theoretical bug, while reviewing code in order to prevent crashes from a driver, which relies on a fully functional restart support.
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index a53feac..74e3529 100644
@@ -807,6 +807,7 @@ void ieee80211_unregister_hw(struct ieee80211_hw *hw)
 
 	rtnl_unlock();
 
+	cancel_work_sync(&local->restart_work);
 	cancel_work_sync(&local->reconfig_filter);
 
 	ieee80211_clear_tx_pending(local);

Is this a stable fix?

Luis