On Sat, 5 Jan 2008, Peter Zijlstra wrote: > > On Sat, 2008-01-05 at 17:53 +0100, Peter Zijlstra wrote: > > On Sat, 2008-01-05 at 18:12 +1100, Herbert Xu wrote: > > > On Fri, Jan 04, 2008 at 09:30:49AM +0100, Ingo Molnar wrote: > > > > > > > > > > [ 1310.670986] ============================================= > > > > > > [ 1310.671690] [ INFO: possible recursive locking detected ] > > > > > > [ 1310.672097] 2.6.24-rc6 #1 > > > > > > [ 1310.672421] --------------------------------------------- > > > > > > [ 1310.672828] FahCore_a0.exe/3692 is trying to acquire lock: > > > > > > [ 1310.673238] (&q->lock){++..}, at: [<c011544b>] > > > > > > __wake_up+0x1b/0x50 > > > > > > [ 1310.673869] > > > > > > [ 1310.673870] but task is already holding lock: > > > > > > [ 1310.674567] (&q->lock){++..}, at: [<c011544b>] > > > > > > __wake_up+0x1b/0x50 > > > > > > [ 1310.675267] > > > > > > [ 1310.675268] other info that might help us debug this: > > > > > > [ 1310.675952] 5 locks held by FahCore_a0.exe/3692: > > > > > > [ 1310.676334] #0: (rcu_read_lock){..--}, at: [<c038b620>] > > > > > > net_rx_action+0x60/0x1b0 > > > > > > [ 1310.677251] #1: (rcu_read_lock){..--}, at: [<c0388d60>] > > > > > > netif_receive_skb+0x100/0x470 > > > > > > [ 1310.677924] #2: (rcu_read_lock){..--}, at: [<c03a7fb2>] > > > > > > ip_local_deliver_finish+0x32/0x210 > > > > > > [ 1310.678460] #3: (clock-AF_INET){-.-?}, at: [<c038164e>] > > > > > > sock_def_readable+0x1e/0x80 > > > > > > [ 1310.679250] #4: (&q->lock){++..}, at: [<c011544b>] > > > > > > __wake_up+0x1b/0x50 > > > > > > The net part might just be a red herring, since the problem is that > > > __wake_up is somehow reentering itself. > > > > /* > > * Perform a safe wake up of the poll wait list. The problem is that > > * with the new callback'd wake up system, it is possible that the > > * poll callback is reentered from inside the call to wake_up() done > > * on the poll wait queue head. The rule is that we cannot reenter the > > * wake up code from the same task more than EP_MAX_POLLWAKE_NESTS times, > > * and we cannot reenter the same wait queue head at all. This will > > * enable to have a hierarchy of epoll file descriptor of no more than > > * EP_MAX_POLLWAKE_NESTS deep. We need the irq version of the spin lock > > * because this one gets called by the poll callback, that in turn is called > > * from inside a wake_up(), that might be called from irq context. > > */ > > > > Seems to suggest that the epoll code can indeed recurse into wakeup. > > > > Davide, Johannes, any ideas? > > Since EP_MAX_POLLWAKE_NESTS < MAX_LOCKDEP_SUBCLASSES we could perhaps do > something like: > > wake_up_nested(..., wake_nests); > > although I'm not quite sure that is correct, my understanding of this > code is still fragile at best.
I remember I talked with Arjan about this time ago. Basically, since 1) you can drop an epoll fd inside another epoll fd 2) callback-based wakeups are used, you can see a wake_up() from inside another wake_up(), but they will never refer to the same lock instance. Think about: dfd = socket(...); efd1 = epoll_create(); efd2 = epoll_create(); epoll_ctl(efd1, EPOLL_CTL_ADD, dfd, ...); epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...); When a packet arrives to the device underneath "dfd", the net code will issue a wake_up() on its poll wake list. Epoll (efd1) has installed a callback wakeup entry on that queue, and the wake_up() performed by the "dfd" net code will end up in ep_poll_callback(). At this point epoll (efd1) notices that it may have some event ready, so it needs to wake up the waiters on its poll wait list (efd2). So it calls ep_poll_safewake() that ends up in another wake_up(), after having checked about the recursion constraints. That are, no more than EP_MAX_POLLWAKE_NESTS, to avoid stack blasting. Never hit the same queue, to avoid loops like: epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...); epoll_ctl(efd3, EPOLL_CTL_ADD, efd2, ...); epoll_ctl(efd4, EPOLL_CTL_ADD, efd3, ...); epoll_ctl(efd1, EPOLL_CTL_ADD, efd4, ...); The code "if (tncur->wq == wq || ..." prevents re-entering the same queue/lock. I don't know how the lockdep code works, so I can't say about wake_up_nested(). Although I have a feeling is not enough in this case. A solution may be to move the call to ep_poll_safewake() (that'd become a simple wake_up()) inside a tasklet or whatever is today trendy for delayed work. But his kinda scares me to be honest, since epoll has already a bunch of places where it could be asynchronously hit (plus performance regression will need to be verified). - Davide -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/