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    SubjectRe: 2.6.21-rc3-mm1
    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
    -
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