On Sunday 16 August 2009 13:32:38 Johannes Berg wrote:
> When removing an interface with nl80211, cfg80211 will
> deadlock in the netdev notifier because we're already
> holding rdev->mtx and try to acquire it again to verify
> the scan has been done.
>
> This bug was introduced by my patch
> "cfg80211: check for and abort dangling scan requests".
>
> To fix this, move the dangling scan request check into
> wiphy_unregister(). This will not be able to catch all
> cases right away, but if the scan problem happens with
> a manual ifdown or so it will be possible to remedy it
> by removing the module/device.
>
> Additionally, add comments about the deadlock scenario.
>
> Reported-by: Christian Lamparter <chunkeey@xxxxxx>
> Signed-off-by: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
Tested-by: Christian Lamparter <chunkeey@xxxxxx>
> ---
> net/wireless/core.c | 32 +++++++++++++++++++-------------
> 1 file changed, 19 insertions(+), 13 deletions(-)
>
> --- wireless-testing.orig/net/wireless/core.c 2009-08-16 13:22:52.000000000 +0200
> +++ wireless-testing/net/wireless/core.c 2009-08-16 13:28:12.000000000 +0200
> @@ -586,9 +586,15 @@ void wiphy_unregister(struct wiphy *wiph
> * get to lock contention here if userspace issues a command
> * that identified the hardware by wiphy index.
> */
> - mutex_lock(&rdev->mtx);
> - /* unlock again before freeing */
> - mutex_unlock(&rdev->mtx);
> + cfg80211_lock_rdev(rdev);
> +
> + if (WARN_ON(rdev->scan_req)) {
> + rdev->scan_req->aborted = true;
> + ___cfg80211_scan_done(rdev);
> + }
> +
> + cfg80211_unlock_rdev(rdev);
> + flush_work(&rdev->scan_done_wk);
>
> cfg80211_debugfs_rdev_del(rdev);
>
> @@ -603,9 +609,7 @@ void wiphy_unregister(struct wiphy *wiph
>
> mutex_unlock(&cfg80211_mutex);
>
> - flush_work(&rdev->scan_done_wk);
> cancel_work_sync(&rdev->conn_work);
> - kfree(rdev->scan_req);
> flush_work(&rdev->event_work);
> }
> EXPORT_SYMBOL(wiphy_unregister);
> @@ -653,6 +657,11 @@ static int cfg80211_netdev_notifier_call
>
> switch (state) {
> case NETDEV_REGISTER:
> + /*
> + * NB: cannot take rdev->mtx here because this may be
> + * called within code protected by it when interfaces
> + * are added with nl80211.
> + */
> mutex_init(&wdev->mtx);
> INIT_LIST_HEAD(&wdev->event_list);
> spin_lock_init(&wdev->event_lock);
> @@ -730,13 +739,11 @@ static int cfg80211_netdev_notifier_call
> #endif
> break;
> case NETDEV_UNREGISTER:
> - cfg80211_lock_rdev(rdev);
> -
> - if (WARN_ON(rdev->scan_req && rdev->scan_req->dev == dev)) {
> - rdev->scan_req->aborted = true;
> - ___cfg80211_scan_done(rdev);
> - }
> -
> + /*
> + * NB: cannot take rdev->mtx here because this may be
> + * called within code protected by it when interfaces
> + * are removed with nl80211.
> + */
> mutex_lock(&rdev->devlist_mtx);
> /*
> * It is possible to get NETDEV_UNREGISTER
> @@ -755,7 +762,6 @@ static int cfg80211_netdev_notifier_call
> #endif
> }
> mutex_unlock(&rdev->devlist_mtx);
> - cfg80211_unlock_rdev(rdev);
> break;
> case NETDEV_PRE_UP:
> if (!(wdev->wiphy->interface_modes & BIT(wdev->iftype)))
>
>
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