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SubjectRe: [2.6.21.1] soft lockup when removing netconsole module
On Tue, May 29, 2007 at 12:56:28AM -0700, Andrew Morton wrote:
> On Sat, 26 May 2007 17:40:12 +0200 Folkert van Heusden <folkert@vanheusden.com> wrote:
>
> > When trying to remove the netconsole module, I got the following kernel
> > output after a while (couple of minutes iirc):
> >
> > [525720.117293] BUG: soft lockup detected on CPU#1!
> > [525720.117353] [<c1004d53>] show_trace_log_lvl+0x1a/0x30
> > [525720.117439] [<c1004d7b>] show_trace+0x12/0x14
> > [525720.117526] [<c1004e75>] dump_stack+0x16/0x18
> > [525720.117613] [<c104dd5b>] softlockup_tick+0xa6/0xc2
> > [525720.117694] [<c1026855>] run_local_timers+0x12/0x14
> > [525720.117738] [<c1026669>] update_process_times+0x72/0xa1
> > [525720.117744] [<c1038673>] tick_sched_timer+0x53/0xb6
> > [525720.117748] [<c1033d62>] hrtimer_interrupt+0x189/0x1e3
> > [525720.117753] [<c100e9e2>] local_apic_timer_interrupt+0x55/0x5b
> > [525720.117761] [<c100ea12>] smp_apic_timer_interrupt+0x2a/0x39
> > [525720.117766] [<c1004a3f>] apic_timer_interrupt+0x33/0x38
> > [525720.117770] [<c120f4b1>] mutex_lock+0x8/0xa
> > [525720.117775] [<c102d2f0>] flush_workqueue+0x2f/0x8f
> > [525720.117780] [<c102d7a0>] cancel_rearming_delayed_workqueue+0x29/0x2b
> > [525720.117785] [<c102d7b1>] cancel_rearming_delayed_work+0xf/0x11
> > [525720.117790] [<c11be143>] netpoll_cleanup+0x75/0xa5
> > [525720.117794] [<f893712d>] cleanup_netconsole+0x17/0x1a [netconsole]
> > [525720.117804] [<c1041f11>] sys_delete_module+0x12f/0x14f
> > [525720.117809] [<c1003f74>] syscall_call+0x7/0xb
> > [525720.117812] =======================
> >
> > Also the rmmod hangs and would not exit even with kill -9. It also
> > sucks up 100% cpu.
>
> Jason recently posted a mystery patch without telling us what problem it
> fixed.
>

To be fair the problem should be known:

http://marc.info/?l=linux-kernel&m=117700287817801&w=2

List: linux-kernel
Subject: Re: [PATCH -mm] workqueue: debug possible endless loop in cancel_rearming_delayed_work
From: Chuck Ebbert <cebbert () redhat ! com>
Date: 2007-04-19 17:07:11
Message-ID: 4627A1BF.8080406 () redhat ! com

> Okay, an easy test for it: insmod netconsole ; rmmod netconsole
>
> In 2.6.20.x it loops forever and cancel_rearming_delayed_work()
> is part of the trace...

I hoped the discussion about cancel_rearming_delayed_work would
reach more people (there was also a patch proposal to add a warning
to the usage comment). But it seem it was not enough...

Of course such a problem should preferably be fixed by somebody who
knows the code (alas I don't know netconsole), to be sure all needed
cancels are still done after this change. I hope Jason's patch is
right but I'm a little surprised I can't see netdev in cc (I'll try
to fix this).

Cheers,
Jarek P.

PS: I'm very sorry for such late response (holidays).

> It looks like you just found it: cancel_rearming_delayed_work() will hang
> if the work isn't actually pending. Please test this:
>
>
> From: Jason Wessel <jason.wessel@windriver.com>
>
> Do not call cancel_rearming_delayed_work() if there is no
> pending work.
>
> Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
> net/core/netpoll.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff -puN net/core/netpoll.c~a net/core/netpoll.c
> --- a/net/core/netpoll.c~a
> +++ a/net/core/netpoll.c
> @@ -784,8 +784,10 @@ void netpoll_cleanup(struct netpoll *np)
> if (atomic_dec_and_test(&npinfo->refcnt)) {
> skb_queue_purge(&npinfo->arp_tx);
> skb_queue_purge(&npinfo->txq);
> - cancel_rearming_delayed_work(&npinfo->tx_work);
> - flush_scheduled_work();
> + if (delayed_work_pending(&npinfo->tx_work)) {
> + cancel_rearming_delayed_work(&npinfo->tx_work);
> + flush_scheduled_work();
> + }
>
> kfree(npinfo);
> }
> _
>
-
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