Consider the following lock and unlock functions from class that puts threads to sleep instead of spin-waiting. Identify the bug(s) in the code. Assume TestAndSet runs atomically.
void lock(lock_t *m) {
while (TestAndSet(&m-guard, 1) == 1)
; // acquire guard lock by spinning
if (m-flag == 0){
m-flag = 1; // lock is acquired
m-guard = 0;
} else {
queue_add(m-q, gettid());
m-guard = 0;
park();
}
}
void unlock(lock_t *m) {
while (TestAndSet(&m-guard, 1) == 1)
; // acquire guard lock by spinning
if (queue_empty(m-q))
m-flag = 0; // let go of lock; no one wants it
else
unpark(queue_remove(m-q)); // hold lock (for next thread!)
m-guard = 0;
}

Question

Consider the following lock and unlock functions from class that puts threads to sleep instead of spin-waiting. Identify the bug(s) in the code. Assume TestAndSet runs atomically. 
 void lock(lock_t *m) {
   while (TestAndSet(&m->guard, 1) == 1)
      ; // acquire guard lock by spinning
   if (m->flag == 0){
      m->flag = 1; // lock is acquired  
       m->guard = 0;  
    } else {
      queue_add(m->q, gettid());  
       m->guard = 0;  
       park();  
     }  
 }  
 void unlock(lock_t *m) {  
    while (TestAndSet(&m->guard, 1) == 1)  
       ; // acquire guard lock by spinning  
    if (queue_empty(m->q))  
       m->flag = 0; // let go of lock; no one wants it  
    else  
       unpark(queue_remove(m->q)); // hold lock (for next thread!)
   m->guard = 0;  
 }

Accepted Answer

In the lock function, a thread may be interrupted after putting its own thread ID as the first ID on the queue but just before calling park and putting itself to sleep. This thread might then miss their wake-up signal and sleep forever.

Answer

There are no bugs in this code.

Answer

In the unlock function, the flag is not always reset to 0, so the lock is never released.

Answer

In the unlock function, a thread may be interrupted after calling unpark but before resetting the guard lock to 0. If a thread awoken from the call to unpark runs immediately it will put itself back to sleep since the guard lock is not available. This time, it will sleep forever since it already used its wakeup signal.

Crowdly

Consider the following lock and unlock functions from class that puts threads to...

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