[Devel] Re: Lockdep splat in cpuset code acquiring alloc_lock

[Paul Menage]

Looks like select_fallback_rq() shouldn't be calling
cpuset_cpus_allowed_locked(), which does a task_lock(), which isn't
IRQ safe. Also, according to its comments that should only be held
with the cpuset callback_mutex held, which seems unlikely from a
softirq handler.

Also, calling cpuset_cpus_allowed_locked(p, &p->cpus_allowed) stomps
on state in p without (AFAICS) synchronization.

The description of commit e76bd8d9850c2296a7e8e24c9dce9b5e6b55fe2f
includes the phrase " I'm fairly sure this works, but there might be a
deadlock hiding" although I think that the lockdep-reported problem is
different than what Rusty had in mind.

Paul

On Wed, Apr 14, 2010 at 1:23 PM, Paul E. McKenney
<paulmck at linux.vnet.ibm.com> wrote:
> Hello!
>
> I hit the following lockdep splat on one of many runs.  Thoughts?
>
>                                                        Thanx, Paul
>
> ------------------------------------------------------------------------
>
> =================================
> [ INFO: inconsistent lock state ]
> 2.6.34-rc3-autokern1 #1
> ---------------------------------
> inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
> swapper/0 [HC0[0]:SC1[1]:HE0:SE0] takes:
>  (&(&p->alloc_lock)->rlock){+.?...}, at: [<c0000000000affd4>] .cpuset_cpus_allowed_locked+0x2c/0xbc
> {SOFTIRQ-ON-W} state was registered at:
>  [<c000000000097ca8>] .lock_acquire+0x5c/0x88
>  [<c000000000570270>] ._raw_spin_lock+0x48/0x70
>  [<c000000000126cec>] .set_task_comm+0x34/0x9c
>  [<c0000000000802f0>] .kthreadd+0x30/0x160
>  [<c000000000026c90>] .kernel_thread+0x54/0x70
> irq event stamp: 782497
> hardirqs last  enabled at (782496): [<c000000000570ebc>] ._raw_spin_unlock_irq+0x38/0x80
> hardirqs last disabled at (782497): [<c000000000050fb4>] .task_rq_lock+0x74/0x130
> softirqs last  enabled at (782478): [<c000000000026acc>] .call_do_softirq+0x14/0x24
> softirqs last disabled at (782493): [<c000000000026acc>] .call_do_softirq+0x14/0x24
>
> other info that might help us debug this:
> 2 locks held by swapper/0:
>  #0:  (&timer){+.-.-.}, at: [<c0000000000712a0>] .run_timer_softirq+0x138/0x298
>  #1:  (rcu_read_lock){.+.+..}, at: [<c000000000054180>] .select_fallback_rq+0xe8/0x1c4
>
> stack backtrace:
> Call Trace:
> [c00000000ffff740] [c000000000010168] .show_stack+0x70/0x184 (unreliable)
> [c00000000ffff7f0] [c0000000000933b8] .print_usage_bug+0x1d4/0x208
> [c00000000ffff8b0] [c000000000093778] .mark_lock+0x38c/0x6f4
> [c00000000ffff960] [c00000000009696c] .__lock_acquire+0x6ac/0x96c
> [c00000000ffffa60] [c000000000097ca8] .lock_acquire+0x5c/0x88
> [c00000000ffffb00] [c000000000570270] ._raw_spin_lock+0x48/0x70
> [c00000000ffffb90] [c0000000000affd4] .cpuset_cpus_allowed_locked+0x2c/0xbc
> [c00000000ffffc20] [c0000000000541bc] .select_fallback_rq+0x124/0x1c4
> [c00000000ffffcd0] [c00000000005ee44] .try_to_wake_up+0x1ac/0x3b4
> [c00000000ffffd80] [c000000000071910] .process_timeout+0x10/0x24
> [c00000000ffffdf0] [c000000000071348] .run_timer_softirq+0x1e0/0x298
> [c00000000ffffed0] [c00000000006a440] .__do_softirq+0x158/0x254
> [c00000000fffff90] [c000000000026acc] .call_do_softirq+0x14/0x24
> [c00000000095b8f0] [c00000000000cd34] .do_softirq+0xb0/0x138
> [c00000000095b990] [c00000000006a680] .irq_exit+0x88/0x100
> [c00000000095ba20] [c0000000000243a8] .timer_interrupt+0x150/0x19c
> [c00000000095bad0] [c000000000003704] decrementer_common+0x104/0x180
> --- Exception: 901 at .raw_local_irq_restore+0x6c/0x80
>    LR = .cpu_idle+0x138/0x20c
> [c00000000095bdc0] [0000000000253400] 0x253400 (unreliable)
> [c00000000095be40] [c0000000000129e8] .cpu_idle+0x138/0x20c
> [c00000000095bed0] [c00000000057a844] .start_secondary+0x3c4/0x404
> [c00000000095bf90] [c000000000008264] .start_secondary_prolog+0x10/0x14
>
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