Linux Kernel Threads with semaphore freeze

Question

I have two kernel threads and I am trying to print from these two threads in alternate fashion. I am using semaphore to sync these two threads.

int kthread_1_function(void *data)
{
    while(1)
    {
        down(&lock);
        pr_alert("In Kernel Thread 1!!!\n");
        up(&lock);

        //kthread_should_stop must be used to check if this thread should be stopped
        if(kthread_should_stop())
            return 1;
    }

    return 0;
}

Similarly I have one more thread which is named as kthread_2_function. In init module, I create these kernel threads ( kthread_run ) and cleanup_module stop these threads ( kthread_stop ).

Following is my observation on various things I tried.

1. When I insert the module, prints are not alternate. I am assuming this is because by the time thread 2 acquires the lock, thread 1 reacquires the lock and prints again.
2. When I use msleep(1000) or mdelay(1000) between down and up function calls in both threads I see alternate prints.
3. When I use msleep(1000) after up function call in both threads again the prints are not alternate.
4. When I use mdelay(1000) after up function call in both threads I see alternate prints but when I remove the module complete system freezes

Can someone please help me understand above observations.

Answer 1

Single semaphore won't give you the interleaving guarantee. You need two semaphores in order to enforce the interleaving. Eg

kthread_1_function:
            down(&lock1);
            pr_alert("In Kernel Thread 1!!!\n");
            up(&lock2);

kthread_2_function:
            down(&lock2);
            pr_alert("In Kernel Thread 2!!!\n");
            up(&lock1);

And you need to unblock semaphores before unload the modules to let the threads shutdown gracefully.