From mboxrd@z Thu Jan  1 00:00:00 1970
From: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: 3.0.10-rt27 arch/arm/kernel/smp.c bug
Date: Tue, 17 Jan 2012 23:32:05 -0500
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To: Antonio Barbalace <antoniob@vt.edu>
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Hi Antonio,

On Fri, 2012-01-13 at 14:17 -0500, Antonio Barbalace wrote:
> I would like to report the following bug that is not still solved in  
> the current 3.0.14 ver.
> 
> [  300.459960] BUG: sleeping function called from invalid context at  
> kernel/rtm5
> [  300.459991] in_atomic(): 1, irqs_disabled(): 128, pid: 9, name: migration/1
> [  300.459991] 1 lock held by migration/1/9:
> [  300.459991]  #0:  (tasklist_lock){++++..}, at: [<c0064fe8>]  
> __cpu_disable+0x0
> [  300.460021] irq event stamp: 1887
> [  300.460052] hardirqs last  enabled at (1886): [<c04e5878>]  
> _raw_spin_unlock_8
> [  300.460052] hardirqs last disabled at (1887): [<c00efd10>]  
> stop_machine_cpu_4
> [  300.460083] softirqs last  enabled at (0): [<c009f628>]  
> copy_process+0x3b4/00
> [  300.460113] softirqs last disabled at (0): [<  (null)>]   (null)
> [  300.460144] [<c00670e4>] (unwind_backtrace+0x0/0xf4) from  
> [<c04e4c88>] (__rt)
> [  300.460174] [<c04e4c88>] (__rt_spin_lock+0x18/0x2c) from  
> [<c04e51e0>] (rt_re)
> [  300.460174] [<c04e51e0>] (rt_read_lock+0x54/0x68) from [<c0064fe8>]  
> (__cpu_d)
> [  300.460235] [<c0064fe8>] (__cpu_disable+0xdc/0x170) from  
> [<c04d4b7c>] (take_)
> [  300.460235] [<c04d4b7c>] (take_cpu_down+0xc/0x30) from [<c00efd28>]  
> (stop_ma)
> [  300.460235] [<c00efd28>] (stop_machine_cpu_stop+0xd8/0x114) from  
> [<c00efad4>)
> [  300.460266] [<c00efad4>] (cpu_stopper_thread+0xb8/0x1ac) from  
> [<c00c29f8>] ()
> [  300.460327] [<c00c29f8>] (kthread+0x88/0x90) from [<c0060914>]  
> (kernel_threa)
> [  300.464385] CPU1: shutdown
> 
> This is due to the following arch/arm/kernel/smp.c @ __cpu_disable code:
> 
> 169         read_lock(&tasklist_lock);
> 170         for_each_process(p) {
> 171                 if (p->mm)
> 172                         cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
> 173         }
> 174         read_unlock(&tasklist_lock);
> 
> I am not a rt expert, do you have any clue on how to solve this problem?

Hmm, I'll need to look at this code deeper. The read_lock() in -rt can
sleep, and this is being called to shutdown a CPU, which I'm sure
disables interrupts along the way.

What did you do to cause this? Does this happen when you take CPU 1
offline?

-- Steve