side effects of calling interruptible_sleep_on_timeout()

[Arun KS]

On Thu, Apr 26, 2012 at 3:48 PM, Shashidhara Shamaiah
<Shashidhara.Shamaiah at mphasis.com> wrote:
> Hi Arun,
>
> My question is how will the scheduler run, if interrupts are disabled on the local CPU. As timer interrupt controls the CPU?

scheduler is nothing but a piece of code( __schedule()) which is just
like any other function.

If interrupts are disabled, you wont be getting the timer interrupts.
So time slice will never get elapsed, which in turn allows current
process to run for ever until it calls schedule again or exits.

But as we figured out that __scheduler is enabling the interrupts back
before we really schedules out. So this problem won't occur in our
Linux kernel.

Thanks,
Arun
>
> Regards
> Shashidhara
>
> -----Original Message-----
> From: Arun KS [mailto:getarunks at gmail.com]
> Sent: Thursday, April 26, 2012 11:36 AM
> To: Shashidhara Shamaiah
> Subject: Re: side effects of calling interruptible_sleep_on_timeout()
>
> Hi,
>
> My code is,
>
> /* disable interrupts and preemption */
> spin_lock_irqsave(&lock, flags);
> /* enable preemption, but interrupt still disabled */
> spin_unlock(&lock);
> /* Now schedule something else */
> schedule_timeout(10 * HZ);
>
> If you see the second step spin_unlock(&lock). This will enable the
> preemtion back.
>
> Arun
>
> On Thu, Apr 26, 2012 at 11:26 AM, Shashidhara Shamaiah
> <Shashidhara.Shamaiah at mphasis.com> wrote:
>> Hi Arun,
>>
>> I think if you run the code on a SMP machine, then the BUG() in schedule() will be triggered as spinlock_irqsave() disables preemption for a SMP.
>>
>> Regards
>> Shashidhara
>>
>> -----Original Message-----
>> From: Arun KS [mailto:getarunks at gmail.com]
>> Sent: Thursday, April 26, 2012 10:41 AM
>> To: Shashidhara Shamaiah
>> Subject: Re: side effects of calling interruptible_sleep_on_timeout()
>>
>> Hello Shashidhara,
>>
>> My machine is UP.
>>
>> thanks,
>> Arun
>>
>> On Thu, Apr 26, 2012 at 10:23 AM, Shashidhara Shamaiah
>> <Shashidhara.Shamaiah at mphasis.com> wrote:
>>> Hi Arun,
>>>
>>> One more question regarding your experiment. Were you running your code on a UP or SMP machine?
>>>
>>> Regards
>>> Shashidhara
>>>
>>> -----Original Message-----
>>> From: kernelnewbies-bounces at kernelnewbies.org [mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of Arun KS
>>> Sent: Thursday, April 26, 2012 10:03 AM
>>> To: Srivatsa S. Bhat
>>> Cc: vinayak menon; Philipp Ittershagen; devendra rawat; kernelnewbies at kernelnewbies.org
>>> Subject: Re: side effects of calling interruptible_sleep_on_timeout()
>>>
>>> Hi Srivatsa,
>>>
>>> On Wed, Apr 25, 2012 at 3:56 PM, Srivatsa S. Bhat
>>> <srivatsa.bhat at linux.vnet.ibm.com> wrote:
>>>> On 04/25/2012 03:36 AM, Philipp Ittershagen wrote:
>>>>
>>>>> Hi Devendra,
>>>>>
>>>>> On Tue, Apr 24, 2012 at 03:24:23PM +0530, devendra rawat wrote:
>>>>>>    Hi,
>>>>>>    A switch driver is causing soft lockup on Montavista Linux Kernel
>>>>>>    2.6.10 system.
>>>>>>    While browsing through the code of the driver. I came across a snippet
>>>>>>    where after disabling the interrupts
>>>>>>    a call is made to interruptible_sleep_on_timeout().
>>>>>>    The code snippet is like
>>>>>>    cli();
>>>>>>    init_waitqueue_head(&queue);
>>>>>>            interruptible_sleep_on_timeout(&queue, USEC_TO_JIFFIES(usec));
>>>>>>            thread_check_signals();
>>>>>>    sti();
>>>>>>    I need to know the side effect of this sort of code, can it be
>>>>>>    responsible for the softlockup of the system ? Its a PowerPC based
>>>>>>    system.
>>>>>
>>>>> you cannot call sleep functions after disabling interrupts, because no
>>>>> interrupt will arrive for the scheduler to see the timeout and resume your
>>>>> task.
>>>>>
>>>>
>>>>
>>>> Yes, that's right. Also, in general, sleeping inside atomic sections (eg.,
>>>> sections with interrupts disabled or preempt disabled) is wrong. There is a
>>>> config option in the kernel that you can use to enable
>>>> sleep-inside-atomic-section-checking (CONFIG_DEBUG_ATOMIC_SLEEP I believe),
>>>> which can help you pin-point such bugs easily.
>>>
>>> I tired an experiment to check this.
>>>
>>> /* disable interrupts and preemption */
>>> spin_lock_irqsave(&lock, flags);
>>> /* enable preemption, but interrupt still disabled */
>>> spin_unlock(&lock);
>>> /* Now schedule something else */
>>> schedule_timeout(10 * HZ);
>>>
>>> But this is not causing any harm. I m able to call schedule with
>>> interrupt disabled and system works fine afterwards.
>>>
>>> So when I looked inside the schedule() function, it checks only
>>> whether preemption is disabled or not. schedule calls  BUG() only if
>>> preemption is disabled and not if interrupts are disabled.
>>>
>>> And AFAIK there is no fuction inside the kernel which tells you that
>>> interrupt are disabled.
>>>
>>> So explantion why system works fine after calling a schedule with
>>> interrupt disabled go here,
>>>
>>> There is a raw_spin_lock_irq(&rq->lock) inside the __schedule() which
>>> in turn calls local_irq_disable().
>>>
>>> local_irq_disable/enable() functions are not nested. We dont have
>>> reference counting.
>>> One call to local_irq_enable is enough to enable multiple calls of
>>> local_irq_disable().
>>>
>>> So my inference is that if you call a schedule with interrupt disable
>>> will not cause any problem. Because schedule function enable it back
>>> before we really schedules out.
>>> But call to schedule() with preemtion disabled will end up in famous
>>> BUG scheduling while atomic.
>>>
>>> NB: Kernel version used is 3.0.15
>>>
>>> Thanks,
>>> Arun
>>>
>>>
>>>>
>>>> Regards,
>>>> Srivatsa S. Bhat
>>>>
>>>>
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