Re: Mysterious CFQ crash and RCU

[Jens Axboe]

On 2011-05-24 00:20, Paul Bolle wrote:
> On Mon, 2011-05-23 at 08:38 -0700, Paul E. McKenney wrote:
>> Running under CONFIG_PREEMPT=y (along with CONFIG_TREE_PREEMPT_RCU=y)
>> could be very helpful in and of itself.  CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
>> can also be helpful.  In post-2.6.39 mainline, it should be possible
>> to set CONFIG_DEBUG_OBJECTS_RCU_HEAD=y without CONFIG_PREEMPT=y, but
>> again, CONFIG_PREEMPT=y can help find problems.
> 
> 0) The first thing I tried (from your suggestions) was
> CONFIG_DEBUG_OBJECTS_RCU_HEAD=y. Given its dependencies (and, well, the
> build system I used) I ended up with:
> 
>     $ grep -e PREEMPT -e RCU /boot/config-2.6.39-0.local3.fc16.i686 |
>     grep -v "^#"
>     CONFIG_TREE_PREEMPT_RCU=y
>     CONFIG_PREEMPT_RCU=y
>     CONFIG_RCU_FANOUT=32
>     CONFIG_PREEMPT_NOTIFIERS=y
>     CONFIG_PREEMPT=y
>     CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
>     CONFIG_DEBUG_PREEMPT=y
>     CONFIG_PROVE_RCU=y
>     CONFIG_SPARSE_RCU_POINTER=y
> 
> It looks like I am unable to trigger the issue we're talking about here
> when using that config.
> 
> 1) For reference, the config of a kernel that does trigger it had:
> 
>     $ grep -e PREEMPT -e RCU /boot/config-2.6.39-0.local2.fc16.i686 |
>     grep -v "^#"
>     CONFIG_TREE_RCU=y
>     CONFIG_RCU_FANOUT=32
>     CONFIG_RCU_FAST_NO_HZ=y
>     CONFIG_PREEMPT_NOTIFIERS=y
>     CONFIG_PREEMPT_VOLUNTARY=y
>     CONFIG_PROVE_RCU=y
>     CONFIG_SPARSE_RCU_POINTER=y
> 
>>> Again CONFIG_TREE_PREEMPT_RCU is available only if PREEMPT=y. So should
>>> we enable preemtion and CONFIG_TREE_PREEMPT_RCU=y and try to reproduce
>>> the issue?
>>
>> Please!
> 
> 2) It appears I can't reproduce with those options enabled (see above).
> 
>> Polling is fine.  Please see attached for a script to poll at 15-second
>> intervals.  Please also feel free to adjust, just tell me what you
>> adjusted.
> 
> And should I now try to run that script on a config that triggers this
> issue (such as the config under 1) above)?

Paul, can we see a dmesg from your running system? Perhaps there's some
dependency on a particular driver or device that makes this easier to
reproduce.

-- 
Jens Axboe

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