struct spinlock {
	uint locked; // Is the lock held?
	// For debugging:
	char *name; // Name of lock.
	struct cpu *cpu; // The cpu holding the lock.
};

// Acquire the lock.
// Loops (spins) until the lock is acquired.
void
acquire(struct spinlock * lk) {
  push_off(); // disable interrupts to avoid deadlock.
  if (holding(lk))
    panic("acquire");
 
  // On RISC-V, sync_lock_test_and_set turns into an atomic swap:
  //   a5 = 1
  //   s1 = &lk->locked
  //   amoswap.w.aq a5, a5, (s1)
  while (__sync_lock_test_and_set( &lk->locked, 1) != 0);
 
  __sync_synchronize();
 
  // Record info about lock acquisition for holding() and debugging.
  lk -> cpu = mycpu();
}
 
// Release the lock.
void
release(struct spinlock * lk) {
  if (!holding(lk))
    panic("release");
 
  lk -> cpu = 0;
  
  __sync_synchronize();
 
  // Release the lock, equivalent to lk->locked = 0.
  // This code doesn't use a C assignment, since the C standard
  // implies that an assignment might be implemented with
  // multiple store instructions.
  // On RISC-V, sync_lock_release turns into an atomic swap:
  //   s1 = &lk->locked
  //   amoswap.w zero, zero, (s1)
  __sync_lock_release( &lk->locked);
 
  pop_off();
}