On Thu, May 03, 2012 at 12:46:02PM -0500, Larry Finger wrote: > On 05/03/2012 12:12 PM, Paul E. McKenney wrote: > >On Thu, May 03, 2012 at 11:54:29AM -0500, Larry Finger wrote: > >>On 05/03/2012 03:47 AM, Catalin Marinas wrote: > >>> > >>>IIUC, Paul suggested that you should use rcu_dereference_check() here > >>>instead as the protected one is not safe in this context. > >> > >>This patch also fails to fix the problem. Did I do what Paul suggested? > > > >That is indeed what I suggested! > > > >What locks does lockdep say are held? > > It varies from instance to instance. I have seen 1, 2, or 3. Those > are the following: > > #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: [<ffffffffa03c1ff0>] > sta_tx_agg_session_timer_expired+0x0/0x2a0 [mac80211] > > > When only 1 lock is held, it is "&tid_tx->session_timer", and that > one is held in every case. I think that means we need to OR it with > the other lockdep_is_held() arguments in the rcu_dereference_xxxxx() > call, but I did not seem to get the syntax right. If any one of several locks (call them a, b, and c) is sufficient to protect the data, and if an rcu_read_lock() also suffices, then something like the following would work: return rcu_dereference_check(sta->ampdu_mlme.tid_tx[tid], lockdep_is_held(&a) || lockdep_is_held(&b) || lockdep_is_held(&c)); But I must defer to the developers and maintainers of that code as to exactly which combinations of locks and RCU are required. The rcu_read_lock_held() is supplied by rcu_dereference_check(), so I am surprised that you got a splat that included rcu_read_lock. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html