Linux Kernel Architecture

Chapter 14: Kernel Activities

list_add(&new->task_list, &head->task_list);

}

Nothing more need be done than to add the new task to the wait list using the standardlist_addlist

function.

add_wait_queue_exclusiveis also available. It works in the same way asadd_wait_queuebut inserts

the process at the queue tail and also sets its flag toWQ_EXCLUSIVE(what is behind this flag is discussed

below).

Another method to put a process to sleep on a wait queue isprepare_to_wait. In addition to the param-

eters required byadd_wait_queue, a task state is required as well:

kernel/wait.c

void fastcall

prepare_to_wait(wait_queue_head_t *q, wait_queue_t *wait, int state)

{

unsigned long flags;

wait->flags &= ~WQ_FLAG_EXCLUSIVE;

spin_lock_irqsave(&q->lock, flags);

if (list_empty(&wait->task_list))

__add_wait_queue(q, wait);

...

set_current_state(state);

spin_unlock_irqrestore(&q->lock, flags);

}

After calling__add_wait_queueas discussed above, the kernel sets the current state of the process to the

state passed toprepare_to_wait.

prepare_to_wait_exclusiveis a variant that sets theWQ_FLAG_EXCLUSIVEflag and appends the wait

queue element to the queue tail.

Two standard methods are available to initialize a wait queue entry:

1. init_waitqueue_entryinitializes a dynamically allocated instance ofwait_queue_t:
   static inline void init_waitqueue_entry(wait_queue_t *q,
   struct task_struct *p)
   {
   q->flags = 0;
   q->private = p;
   q->func = default_wake_function;
   }
   

default_wake_functionis just a parameter conversion front end that attempts to wake the

process using thetry_to_wake_upfunction described in Chapter 2.