On 01/22/2015 11:02 PM, Sasha Levin wrote:
> On 01/22/2015 10:51 PM, Paul E. McKenney wrote:
>> On Thu, Jan 22, 2015 at 10:29:01PM -0500, Sasha Levin wrote:
>>>> On 01/21/2015 07:43 PM, Paul E. McKenney wrote:
>>>>>> On Wed, Jan 21, 2015 at 10:44:57AM -0500, Sasha Levin wrote:
>>>>>>>> On 01/20/2015 09:57 PM, Paul E. McKenney wrote:
>>>>>>>>>>>>>> So RCU believes that an RCU read-side critical section that
>>>>>>>>>>>>>> ended within
>>>>>>>>>>>>>>>>>> an interrupt handler (in this case, an hrtimer) somehow got
>>>>>>>>>>>>>>>>>> preempted.
>>>>>>>>>>>>>>>>>> Which is not supposed to happen.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Do you have CONFIG_PROVE_RCU enabled? If not, could you
>>>>>>>>>>>>>>>>>> please enable it
>>>>>>>>>>>>>>>>>> and retry?
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> I did have CONFIG_PROVE_RCU, and didn't see anything else
>>>>>>>>>>>>>> besides what I pasted here.
>>>>>>>>>> OK, fair enough. I do have a stack of RCU CPU stall-warning changes
>>>>>>>>>> on
>>>>>>>>>> their way in, please see v3.19-rc1..630181c4a915 in -rcu, which is
>>>>>>>>>> at:
>>>>>>>>>>
>>>>>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
>>>>>>>>>>
>>>>>>>>>> These handle the problems that Dave Jones, yourself, and a few others
>>>>>>>>>> located this past December. Could you please give them a spin?
>>>>>>>>
>>>>>>>> They seem to be a part of -next already, so this testing already
>>>>>>>> includes them.
>>>>>>>>
>>>>>>>> I seem to be getting them about once a day, anything I can add to
>>>>>>>> debug it?
>>>>>>
>>>>>> Could you please try reproducing with the following patch?
>>>>
>>>> Yes, and I've got mixed results. It reproduced, and all I got was:
>>>>
>>>> [ 717.645572] ===============================
>>>> [ 717.645572] [ INFO: suspicious RCU usage. ]
>>>> [ 717.645572] 3.19.0-rc5-next-20150121-sasha-00064-g3c37e35-dirty #1809
>>>> Tainted: G W
>>>> [ 717.645572] -------------------------------
>>>> [ 717.645572] kernel/rcu/tree_plugin.h:337 rcu_read_unlock() from irq or
>>>> softirq with blocking in critical section!!!
>>>> [ 717.645572] !
>>>> [ 717.645572]
>>>> [ 717.645572] other info that might help us debug this:
>>>> [ 717.645572]
>>>> [ 717.645572]
>>>> [ 717.645572] rcu_scheduler_active = 1, debug_locks = 1
>>>> [ 717.645572] 3 locks held by trinity-c29/16497:
>>>> [ 717.645572] #0: (&sb->s_type->i_mutex_key){+.+.+.}, at:
>>>> [<ffffffff81bec373>] lookup_slow+0xd3/0x420
>>>> [ 717.645572] #1:
>>>> [hang]
>>>>
>>>> So the rest of the locks/stack trace didn't get printed, nor the
>>>> pr_alert() which
>>>> should follow that.
>>>>
>>>> I've removed the lockdep call and will re-run it.
>> Thank you! You are keeping the pr_alert(), correct?
>
> Yup, just the lockdep call goes away.
Okay, this reproduced faster than I anticipated:
[ 786.160131] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[ 786.239513] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[ 786.240503] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[ 786.242575] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[ 786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[ 786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[ 786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[ 786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[ 786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
It seems like the WARN_ON_ONCE was hiding the fact it actually got hit couple
of times in a very short interval. Maybe that would also explain lockdep
crapping
itself.
Thanks,
Sasha
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