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Subject: Re: [2.6.33-rc5] Weird deadlock when shutting down
From: Johannes Berg <johannes@sipsolutions.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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On Thu, 2010-02-18 at 08:31 -0800, Linus Torvalds wrote:

> > > halt/4071 is trying to acquire lock:
> > >  (s_active){++++.+}, at: [<c0000000001ef868>]
> > > .sysfs_addrm_finish+0x58/0xc0
> > >=20
> > > but task is already holding lock:
> > >  (&per_cpu(cpu_policy_rwsem, cpu)){+.+.+.}, at:
> [<c0000000004cd6ac>]
> > > .lock_policy_rwsem_write+0x84/0xf4
> > >=20
> > > which lock already depends on the new lock.
>=20
> You don't have a full backtrace for these things?

No, it deadlocks right there, unfortunately.

> We've had lots of trouble with the cpu governors, and I suspect the=20
> problem isn't new, but the lockdep warning is likely new (see commit=20
> 846f99749ab68bbc7f75c74fec305de675b1a1bf: "sysfs: Add lockdep
> annotations=20
> for the sysfs active reference").
>=20
> 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.

Well, it used to not deadlock and actually shut down the machine :) So
in that sense it's definitely new. It might have printed a lockdep
warning before, which you wouldn't normally see since the machine turns
off right after this.

> I suspect it is G5-specific (or specific to whatever CPU frequency
> code=20
> that gets used there), since I think we'd have had lots of reports if
> this=20
> happened on x86.

Yeah, that's puzzling me as well.

johannes

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