Re: Suspicious RCU usage in mac80211

[Larry Finger <Larry.Finger@lwfinger.net>]

On 05/02/2012 12:07 PM, Johannes Berg wrote:
> On Wed, 2012-05-02 at 11:00 +0100, Catalin Marinas wrote:
>
>>> Your patch does not help. I still get the following dump in the log:
>
> The patch is also wrong, we hold the mutex there and can't hold RCU read
> lock.
>
>>> #0:  (scan_mutex){+.+...}, at: [<ffffffff8113b0d6>] kmemleak_scan_thread+0x56/0xd0
>>> #1:  (&tid_tx->session_timer){+.-...}, at: [<ffffffff8104853a>]
>>> run_timer_softirq+0xfa/0x6e0
>>> #2:  (rcu_read_lock){.+.+..}, at: [<ffffffffa0449ff0>]
>>> sta_tx_agg_session_timer_expired+0x0/0x2a0 [mac80211]
>>>
>>> stack backtrace:
>>> Pid: 622, comm: kmemleak Not tainted 3.4.0-rc5-wl+ #287
>>> Call Trace:
>>>    <IRQ>   [<ffffffff8109309d>] lockdep_rcu_suspicious+0xfd/0x130
>>>    [<ffffffffa044a1cf>] sta_tx_agg_session_timer_expired+0x1df/0x2a0 [mac80211]
>>>    [<ffffffffa0449ff0>] ? ieee80211_start_tx_ba_session+0x450/0x450 [mac80211]
>>>    [<ffffffff810485c5>] run_timer_softirq+0x185/0x6e0
>>>
>>> As kmemleak seems to be involved, I have added Catalin Marinas to the Cc list.
>
>> Looking at the code and the logs, ieee80211_start_tx_ba_session() calls
>
> I'm almost certain that ieee80211_start_tx_ba_session() is a bogus
> calltrace entry, since we're in a timer and that's not called from a
> timer.
>
>> rcu_dereference_protected_tid_tx() which calls
>> rcu_dereference_protected() with the (lockdep_is_held(&sta->lock) ||
>> lockdep_is_held(&sta->ampdu_mlme.mtx)) condition which is false. As the
>> kernel log says, none of these locks are held, hence the warning.
>
> So does that just mean we need to add rcu_read_lock_held() to the
> conditions? I thought that wasn't necessary? +Paul.

If you meant the following

Index: wireless-testing-new/net/mac80211/sta_info.h
===================================================================
--- wireless-testing-new.orig/net/mac80211/sta_info.h
+++ wireless-testing-new/net/mac80211/sta_info.h
@@ -453,7 +453,8 @@ rcu_dereference_protected_tid_tx(struct
  {
         return rcu_dereference_protected(sta->ampdu_mlme.tid_tx[tid],
                                          lockdep_is_held(&sta->lock) ||
-                                        lockdep_is_held(&sta->ampdu_mlme.mtx));
+                                        lockdep_is_held(&sta->ampdu_mlme.mtx) ||
+                                        rcu_read_lock_held());
  }

  #define STA_HASH_SIZE 256

It does not help.

Larry