linux-kernel - Re: [2.6.33-rc5] Weird deadlock when shutting down

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Message-ID: <2375c9f91002192046u5105db8j4b8a63647d9220ce@mail.gmail.com>
Date:	Sat, 20 Feb 2010 12:46:48 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Johannes Berg <johannes@...solutions.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [2.6.33-rc5] Weird deadlock when shutting down

On Fri, Feb 19, 2010 at 12:31 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>
> On Thu, 18 Feb 2010, Johannes Berg wrote:
>> >
>> > Basically, the machine deadlocks right after printing the following
>> > when doing a shutdown:
>> >
>> > halt/4071 is trying to acquire lock:
>> >  (s_active){++++.+}, at: [<c0000000001ef868>]
>> > .sysfs_addrm_finish+0x58/0xc0
>> >
>> > but task is already holding lock:
>> >  (&per_cpu(cpu_policy_rwsem, cpu)){+.+.+.}, at: [<c0000000004cd6ac>]
>> > .lock_policy_rwsem_write+0x84/0xf4
>> >
>> > which lock already depends on the new lock.
>
> You don't have a full backtrace for these things?
>
> We've had lots of trouble with the cpu governors, and I suspect the
> problem isn't new, but the lockdep warning is likely new (see commit
> 846f99749ab68bbc7f75c74fec305de675b1a1bf: "sysfs: Add lockdep annotations
> for the sysfs active reference").
>
> So it is likely to be an old issue that (a) now gets warned about and (b)
> might have had timing changes enough to trigger it.
>

Right.

This is a real deadlock case found by lockdep added to s_active.

The problem is that we did kobject_put(&data->kobj) while holding policy_rwsem
which is used to protect 'data'. It is not so easy to fix this,
probably we need to
do more work on cpufreq code.

Thanks.
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