Down_interruptible on Linux

Question

I find it difficult to understand the following LDD3 statement. "down_interruptible - this allows a user-space process that expects a semaphore to be interrupted by the user."

A user space application will not direct a down_interruptible call. Suppose a device driver is running, and the application goes into sleep mode with a device driver called by calling down_interruptible. Now, as a signal to the user space of the application, it calls the application from sleep, because its the device driver that called down_interruptible, not the application.

Someone please clarify this to me.

Answer 1

Any device driver does not start independently; the device driver starts on behalf of the process through system calls.

Suppose that any device driver calls down_interruptible();, this means that if the semaphore is unavailable, the corresponding process will be placed on the semaphore waiting queue. And the state of the task will be changed to TASK_INTERRUPTIBLE, and the scheduler is called to start any other process. Now the sleep process can be awakened either by an event for its expectation (semaphore), or by a signal.

Example: kill -SIGINT <pid>will cause the process to change its state to TASK_RUNNINGand add a process to start the queue.

Here it is the pseudo code of the wait queue, as the process is waiting for an event.

/* ‘q’ is the wait queue we wish to sleep on */
DEFINE_WAIT(wait);
add_wait_queue(q, &wait);
while (!condition) /* condition is the event that we are waiting for */
{
    prepare_to_wait(&q, &wait, TASK_INTERRUPTIBLE);
    if (!signal_pending(current))
    {
        schedule();
        continue;
    }
    ret = -ERESTARTSYS; 
}
finish_wait(&q, &wait);

, . , , -ERESTARTSYS, .