Re: Suspicious RCU usage in mac80211

[Johannes Berg <johannes@sipsolutions.net>]

On Mon, 2012-04-09 at 22:19 -0500, Larry Finger wrote:
> When running kernel 3.4-rc2 from wireless testing, I got the following logged:
> 
> [ 2299.344437] ===============================
> [ 2299.344443] [ INFO: suspicious RCU usage. ]
> [ 2299.344452] 3.4.0-rc2-wl+ #222 Not tainted
> [ 2299.344458] -------------------------------
> [ 2299.344466] net/mac80211/sta_info.h:449 suspicious rcu_dereference_protected() usage!
> [ 2299.344472]
> [ 2299.344474] other info that might help us debug this:
> [ 2299.344477]
> [ 2299.344484]
> [ 2299.344486] rcu_scheduler_active = 1, debug_locks = 0
> [ 2299.344493] 1 lock held by swapper/1/0:
> [ 2299.344499]  #0:  (&tid_tx->session_timer){+.-...}, at: [<ffffffff8104850a>] 
> run_timer_softirq+0xfa/0x6e0
> [ 2299.344533]
> [ 2299.344535] stack backtrace:
> [ 2299.344544] Pid: 0, comm: swapper/1 Not tainted 3.4.0-rc2-wl+ #222
> [ 2299.344551] Call Trace:
> [ 2299.344557]  <IRQ>  [<ffffffff81092fdd>] lockdep_rcu_suspicious+0xfd/0x130
> [ 2299.344651]  [<ffffffffa05db673>] sta_tx_agg_session_timer_expired+0xe3/0x100 
> [mac80211]

That's odd:

static inline struct tid_ampdu_tx *
rcu_dereference_protected_tid_tx(struct sta_info *sta, int tid)
{
        return rcu_dereference_protected(sta->ampdu_mlme.tid_tx[tid],
                                         lockdep_is_held(&sta->lock) ||
                                         lockdep_is_held(&sta->ampdu_mlme.mtx));
} 

sta_tx_agg_session_timer_expired calls ieee80211_stop_tx_ba_session
which does:

        spin_lock_bh(&sta->lock);
        tid_tx = rcu_dereference_protected_tid_tx(sta, tid);


So why would the message happen? Strange.

johannes