Re: [PATCH 1/5] drivers/misc: Support for RF interface device framework

[pankaj chauhan <pankaj.chauhan@freescale.com>]

On 6/19/2013 5:01 PM, Lars-Peter Clausen wrote:
> On 06/18/2013 09:44 AM, Akhil Goyal wrote:
> [...]
>>>> +    /*
>>>> +     * Spin_locks are changed to mutexes if PREEMPT_RT is enabled,
>>>> +     * i.e they can sleep. This fact is problem for us because
>>>> +     * add_wait_queue()/wake_up_all() takes wait queue spin lock.
>>>> +     * Since spin lock can sleep with PREEMPT_RT, wake_up_all() can not be
>>>> +     * called from rf_notify_dl_tti (which is called in interrupt context).
>>>> +     * As a workaround, wait_q_lock is used for protecting the wait_q and
>>>> +     * add_wait_queue_locked()/ wake_up_locked() functions of wait queues
>>>> +     * are used.
>>>> +     */
>>>> +    raw_spin_lock_irqsave(&rf_dev->wait_q_lock, flags);
>>>> +    __add_wait_queue_tail_exclusive(&rf_dev->wait_q,&wait);
>>>> +    raw_spin_unlock_irqrestore(&rf_dev->wait_q_lock, flags);
>>>> +    set_current_state(TASK_INTERRUPTIBLE);
>>>> +    /*Now wait here, tti notificaion will wake us up*/
>>>> +    schedule();
>>>> +    set_current_state(TASK_RUNNING);
>>>> +    raw_spin_lock_irqsave(&rf_dev->wait_q_lock, flags);
>>>> +    __remove_wait_queue(&rf_dev->wait_q,&wait);
>>>> +    raw_spin_unlock_irqrestore(&rf_dev->wait_q_lock, flags);
>>>
>>> This is not a proper method of waiting for an event. Why can't you
>>> use wait_event() here?
>> wait_event() is internally calling spin_lock_irqsave() and this function
>> will be called in hard IRQ context with PREEMPT_RT enabled(IRQF_NODELAY
>> set). So wait_event cannot be used.
>> This problem can be solved if we can get the following patch applied on the
>> tree.
>> https://patchwork.kernel.org/patch/2161261/
>>
>>>
>>> The explanation about the interrupt handler seems incorrect, since PREEMPT_RT
>>> also turns interrupt handlers into threads.
>> The interrupt handler has real time requirement and thus running in HARDIRQ
>> context with flag IRQF_NODELAY. We get this interrupt in every millisecond.
>
> But if you are running in HARDIRQ context the whole sequence doesn't make
> much sense at all, since you won't be able to sleep and wait for the event.
>

This function for adding process to rf_dev->wait_q runs in process 
context, but the function (dl_notify_tti())which wakes process up from 
this queue runs in HARDIRQ context, that's why we could not use 
wait_event() or spin_lock() here.

thanks,
pankaj