Re: [mmotm][BUG] lockdep warning block I/O (Was Re: mmotm 2009-08-27-16-51 uploaded

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 3 Sep 2009 14:28:36 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Tue, 1 Sep 2009 18:07:17 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> 
> > 
> > Here is mmont-Aug27's lockdep wanring. This was printed out when oom-kill happens.
> > I'm sorry if already fixed.
> 
> My life's project is to hunt down the guy who invented mail client
> wordwrapping, set him on fire then dance on his ashes.
> 
Hmm, I should write a script to cut "Sep 1 ,,,,, : [.....]"...



> > =
> > Sep  1 18:01:16 localhost kernel: [ 3012.503035] ======================================================
> > Sep  1 18:01:16 localhost kernel: [ 3012.503039] [ INFO: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected ]
> > Sep  1 18:01:16 localhost kernel: [ 3012.503042] 2.6.31-rc7-mm1 #3
> > Sep  1 18:01:16 localhost kernel: [ 3012.503049] ------------------------------------------------------
> > Sep  1 18:01:16 localhost kernel: [ 3012.503052] kblockd/7/350 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
> > Sep  1 18:01:16 localhost kernel: [ 3012.503058]  (bdev_lock){+.+...}, at: [<ffffffff811458c7>] nr_blockdev_pages+0x1
> > 7/0x80
> > Sep  1 18:01:16 localhost kernel: [ 3012.503069]
> > Sep  1 18:01:16 localhost kernel: [ 3012.503070] and this task is already holding:
> > Sep  1 18:01:16 localhost kernel: [ 3012.503075]  (&q->__queue_lock){..-.-.}, at: [<ffffffff811e9ff8>] cfq_kick_queue
> > +0x28/0x50
> > Sep  1 18:01:16 localhost kernel: [ 3012.503083] which would create a new lock dependency:
> > Sep  1 18:01:16 localhost kernel: [ 3012.503087]  (&q->__queue_lock){..-.-.} -> (bdev_lock){+.+...}
> > Sep  1 18:01:16 localhost kernel: [ 3012.503100]
> 
> I'd say the core problem here is that __alloc_pages_slowpath() is
> calling show_mem().  Because show_mem() is a "high level" function which
> takes "high level" locks.  ie: bdev_lock.
> 
> It's inappropriate that alloc_pages() is assuming that it is safe to
> call show_mem() from all contexts in which alloc_pages() might be
> called.
> 
> That show_mem() call has been there since 2005, so I don't know what
> caused this to be revealed now.
> 
> It's not at all a serious bug and the chances of us deadlocking the
> kernel here are close to zero.  An appropriate fix would be to replace
> that show_mem() call with something which can be safely called from all
> contexts in which the page allocator can be called.
> 
ok, I'll study this path.

Thanks,
-Kame

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