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Subject: Re: 2.6.24-rc6: possible recursive locking detected
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Ingo Molnar <mingo@elte.hu>, "Rafael J. Wysocki" <rjw@sisk.pl>,
       Christian Kujau <lists@nerdbynature.de>, linux-kernel@vger.kernel.org,
       jfs-discussion@lists.sourceforge.net,
       Davide Libenzi <davidel@xmailserver.org>,
       Johannes Berg <johannes@sipsolutions.net>,
       Oleg Nesterov <oleg@tv-sign.ru>
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Date: Sat, 05 Jan 2008 17:53:36 +0100
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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?