Lab4: Lock

Armv8-A provides ldxr and stxr for exclusive access. You can either use compiler’s built-in function or hand written assembly. However, you need to enable MMU and data cache before using the ldxr instruction in real rpi3. So, you can now use a workaround such as disable preemption in real rpi3 or just give it a trial in QEMU which doesn’t have to enable MMU for ldxr.

Requirement

Implement mutex_lock, mutex_unlock. If task fail to acquire the lock, it would go to sleep and context switch to other tasks.

Implementation

以 disable interrupt 方法為主，後續啟用 MMU 才會使用 exclusive access 的指令。

Demo

process 試著取得 lock 後馬上 scheule() 給下一個 process 去取得 lock，拿到 lock 後該 process 結束。一次只會有一個 process 拿到 lock，因此可見到 “Got lock” 交錯印出。

#include <include/lock.h>
mutex_t shared_lock;

void task1()
{
    while (1) {
        printk("[PID %d] Kernel task 1...try to get lock\n", do_get_taskid());
        int ret = mutex_trylock(&shared_lock);
        enable_irq();  // Enable IRQ if it returns from IRQ handler
        schedule();
        if (ret == 0) {
            printk("[PID %d] Got lock, exit...\n", do_get_taskid());
            mutex_unlock(&shared_lock);
            do_exit();
        } else if (ret == EBUSY) {
            printk("[PID %d] Already locked.\n", do_get_taskid());
        }
    }
}

void task2()
{
    while (1) {
        printk("[PID %d] Kernel task 2...try to get lock\n", do_get_taskid());
        int ret = mutex_trylock(&shared_lock);
        schedule();
        enable_irq();
        if (ret == 0) {
            printk("[PID %d] Got lock, exit...\n", do_get_taskid());
            mutex_unlock(&shared_lock);
            do_exit();
        } else if (ret == EBUSY) {
            printk("[PID %d] Already locked.\n", do_get_taskid());
        }
    }
}

Reference

罗玉平： 关于ARM Linux原子操作的底层支持