Re: 2.6.21-rc3-mm1

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Mar 09, 2007 at 06:18:51PM -0800, Andrew Morton wrote:
> > On Thu, 08 Mar 2007 21:50:29 +0100 Michal Piotrowski <michal.k.k.piotrowski@gmail.com> wrote:
> > Andrew Morton napisał(a):
> > > Temporarily at
> > > 
> > >   http://userweb.kernel.org/~akpm/2.6.21-rc3-mm1/
> > > 
> > > Will appear later at
> > > 
> > >   ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.21-rc3/2.6.21-rc3-mm1/
> > > 
> > 
> > cpu_hotplug (AutoTest) hangs at this
> > 
> > =============================================
> > [ INFO: possible recursive locking detected ]
> > 2.6.21-rc3-mm1 #2
> > ---------------------------------------------
> > sh/7213 is trying to acquire lock:
> >  (sched_hotcpu_mutex){--..}, at: [<c033883a>] mutex_lock+0x1c/0x1f
> > 
> > but task is already holding lock:
> >  (sched_hotcpu_mutex){--..}, at: [<c033883a>] mutex_lock+0x1c/0x1f
> > 
> > other info that might help us debug this:
> > 4 locks held by sh/7213:
> >  #0:  (cpu_add_remove_lock){--..}, at: [<c033883a>] mutex_lock+0x1c/0x1f
> >  #1:  (sched_hotcpu_mutex){--..}, at: [<c033883a>] mutex_lock+0x1c/0x1f
> >  #2:  (cache_chain_mutex){--..}, at: [<c033883a>] mutex_lock+0x1c/0x1f
> >  #3:  (workqueue_mutex){--..}, at: [<c033883a>] mutex_lock+0x1c/0x1f
> 
> That's pretty useless, isn't it?  We need to know the mutex_lock() caller
> here.
> 
> > stack backtrace
> >  [<c0105256>] show_trace_log_lvl+0x1a/0x2f
> >  [<c010597b>] show_trace+0x12/0x14
> >  [<c0105a3d>] dump_stack+0x16/0x18
> >  [<c013fc73>] __lock_acquire+0x1aa/0xceb
> >  [<c014082d>] lock_acquire+0x79/0x93
> >  [<c03385dc>] __mutex_lock_slowpath+0x107/0x349
> >  [<c033883a>] mutex_lock+0x1c/0x1f
> >  [<c011d924>] sched_getaffinity+0x14/0x91
> >  [<c015796d>] __synchronize_sched+0x11/0x5f
> >  [<c011d257>] detach_destroy_domains+0x2c/0x30
> >  [<c011fc1a>] update_sched_domains+0x27/0x3a
> >  [<c012fe7a>] notifier_call_chain+0x2b/0x4a
> >  [<c012fec6>] __raw_notifier_call_chain+0x19/0x1e
> >  [<c0145756>] _cpu_down+0x70/0x282
> >  [<c014598e>] cpu_down+0x26/0x38
> >  [<c0272714>] store_online+0x27/0x5a
> >  [<c026f610>] sysdev_store+0x20/0x25
> >  [<c01b7a8e>] sysfs_write_file+0xc1/0xe9
> >  [<c0180052>] vfs_write+0xd1/0x15a
> >  [<c0180682>] sys_write+0x3d/0x72
> >  [<c0104270>] syscall_call+0x7/0xb
> > 
> > l *0xc033883a
> > 0xc033883a is in mutex_lock (/mnt/md0/devel/linux-mm/kernel/mutex.c:92).
> > 87              /*
> > 88               * The locking fastpath is the 1->0 transition from
> > 89               * 'unlocked' into 'locked' state.
> > 90               */
> > 91              __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
> > 92      }
> > 93
> > 94      EXPORT_SYMBOL(mutex_lock);
> > 95
> > 96      static void fastcall noinline __sched
> > 
> > I didn't test other -mm's with this test.
> > 
> > http://www.stardust.webpages.pl/files/tbf/bitis-gabonica/2.6.21-rc3-mm1/console.log
> > http://www.stardust.webpages.pl/files/tbf/bitis-gabonica/2.6.21-rc3-mm1/mm-config
> 
> I can't immediately spot the bug.  Probably it's caused by rcu-preempt's
> changes to synchronize_sched(): that function now does a heap more than it
> used to, including taking sched_hotcpu_muex.
> 
> So, what to do about this.  Paul, I'm thinking that I should drop
> rcu-preempt for now - I don't think we ended up being able to identify any
> particular benefit which it brings to current mainline, and I suspect that
> things will become simpler if/when we start using the process freezer for
> CPU hotplug.

It certainly makes sense for Michal to try backing out rcu-preempt using
your broken-out list of patches.  If that makes the problem go away,
then I would certainly have a hard time arguing with you.  We are working
on getting measurements showing benefit of rcu-preempt, but aren't there
yet.

						Thanx, Paul