linux-kernel - FYI: BUG: deadlock workqueues + OOM

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Date:	Fri, 21 Jan 2011 16:37:24 -0600
From:	Linas Vepstas <linasvepstas@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: FYI: BUG: deadlock workqueues + OOM

I've been working on a new arch (patches to be submitted "real soon now")
and have started seeing a deadlock in the workqueues.  This email is "FYI",
as I don't have much in the way of good evidence yet, but it seems like an
arch-indep bug so I thought I'd report it :-)

kernel: linux-2.6.37-rc8
system: 768K RAM, 4-way cpu, rootfs on NFS, no local block storage, no swap.

scenario: run a "mempig" that occasionally triggers the OOM killer, while
also running a pthread-create bomb (like fork-bomb but for threads) (but
each thread returns immediately).

Deadlock: two cpu's in idle loop, two cpu's spinning on spinlock in
kernel/workqueue.c, interrupts disabled.  A pair of "typical" stack
traces below:

c0288484 _raw_spin_lock_irqsave
c0041224 __queue_work  -- kernel/workqueue.c
spin_lock_irqsave(&gcwq->lock, flags);
c0041500 queue_work_on
c025d7d0 xprt_force_disconnect
xs_tcp_write_space
tcp_fin
svc_drop
tcp_rcv_established
 ... etc.

c02883c4 _raw_spin_lock_irq
c0042d80 start_flush_work kernel/workqueue.c
c00435d0 flush_work
c01aed54 n_tty_read
c01a99cc tty_read
... etc.

The precise stack traces vary, but they always end up with one cpu
in start_flush_work()  and the other in __queue_work()

I was wondering if this reminds anyone of anything.  I'll provide
more if/when I narrow it down.

--linas
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