side effects of calling interruptible_sleep_on_timeout()

[Arun KS]

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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