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

From: pankaj chauhan
Date: Thu Jun 20 2013 - 06:48:28 EST

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

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.


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