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Subject: Re: [PATCH tip/core/urgent 1/7] rcu: decrease rcu_report_exp_rnp
 coupling with scheduler
From: Peter Zijlstra <peterz@infradead.org>
To: Ed Tomlinson <edt@aei.ca>
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Date: Wed, 20 Jul 2011 14:35:20 +0200
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On Wed, 2011-07-20 at 07:23 -0400, Ed Tomlinson wrote:
> [    3.553004]  [<ffffffff8104b06a>] warn_slowpath_null+0x1a/0x20
> [    3.553004]  [<ffffffff810bb479>] __rcu_read_unlock+0xc9/0x120
> [    3.553004]  [<ffffffff8103fed8>] cpuacct_charge+0xc8/0xe0
> [    3.553004]  [<ffffffff8103fe58>] ? cpuacct_charge+0x48/0xe0
> [    3.553004]  [<ffffffff810326b7>] ? task_of+0x97/0xd0
> [    3.553004]  [<ffffffff81040ef5>] update_curr+0x1a5/0x210
> [    3.553004]  [<ffffffff81576d78>] ? preempt_schedule_irq+0x68/0xa0
> [    3.553004]  [<ffffffff810419e0>] put_prev_task_fair+0x110/0x120
> [    3.553004]  [<ffffffff81575f3a>] schedule+0x1da/0xc50
> [    3.553004]  [<ffffffff81576d72>] ? preempt_schedule_irq+0x62/0xa0
> [    3.553004]  [<ffffffff81576d78>] preempt_schedule_irq+0x68/0xa0
> [    3.553004]  [<ffffffff8157a316>] retint_kernel+0x26/0x30
> [    3.553004]  [<ffffffff810da494>] ? ftrace_likely_update+0x14/0x20
> [    3.553004]  [<ffffffff810bb4ab>] __rcu_read_unlock+0xfb/0x120
> [    3.553004]  [<ffffffff810f8190>] find_get_page+0x170/0x190 

Ok, so we're running some task that does rcu_read_unlock(), right in the
middle of __rcu_read_unlock() we get preempted, the scheduler calls
rcu_note_context_switch()->rcu_preempt_note_context_switch() which sets
->rcu_read_unlock_special |= UNLOCK_BLOCKED.

Then before finishing the context switch, the cpuacct muck uses rcu, and
its rcu_read_unlock() triggers __rcu_read_unlock() and goes bang.

That rcu usage isn't new, that's been there since March 2009.

AFAICT even .39 should suffer from this particular issue, or am I
missing something here.. Paul?