Timer interrupt in kernel mode and then yield question in xv6

Question

I'm studying s081 2021 OS course, and I encountered a problem. The process assumes the following.

Assume that a clock interrupt occurs in the process in user mode, and then it will enter trap.c, at this time, the interrupt will be disabled, but the clock interrupt in machine mode will not be blocked. Then in trap.c we will enter yield(), in yield() function, we execute acquire(&p->lock) and then obtain the lock of the current process. But at this time, a clock interrupt occurred in kernel mode, and we will go to kerneltrap(). In kerneltrap(), we will enter yield() again and try to acquire the lock of the current process in acquire(). In acquire(), we will check hold(lk), found that the current CPU has acquired the lock of the process, thus triggering panic, in panic, xv6 will enter an infinite loop.

But in actual xv6, the process does not suddenly fall into an infinite loop. I don't know where my understanding above is wrong.

void
usertrap(void)
{
  int which_dev = 0;

  if((r_sstatus() & SSTATUS_SPP) != 0)
    panic("usertrap: not from user mode");

  // send interrupts and exceptions to kerneltrap(),
  // since we're now in the kernel.
  w_stvec((uint64)kernelvec);

  struct proc *p = myproc();
  
  // save user program counter.
  p->trapframe->epc = r_sepc();
  
  if(r_scause() == 8){
    // system call

    if(p->killed)
      exit(-1);

    // sepc points to the ecall instruction,
    // but we want to return to the next instruction.
    p->trapframe->epc += 4;

    // an interrupt will change sstatus &c registers,
    // so don't enable until done with those registers.
    intr_on();

    syscall();
  } else if((which_dev = devintr()) != 0){
    // ok
  } else {
    printf("usertrap(): unexpected scause %p pid=%d\n", r_scause(), p->pid);
    printf("            sepc=%p stval=%p\n", r_sepc(), r_stval());
    p->killed = 1;
  }

  if(p->killed)
    exit(-1);

  // give up the CPU if this is a timer interrupt.
  if(which_dev == 2)
    yield();

  usertrapret();
}

void
yield(void)
{
  struct proc *p = myproc();
  acquire(&p->lock);
  p->state = RUNNABLE;
  sched();
  release(&p->lock);
}

void 
kerneltrap()
{
  int which_dev = 0;
  uint64 sepc = r_sepc();
  uint64 sstatus = r_sstatus();
  uint64 scause = r_scause();
  
  if((sstatus & SSTATUS_SPP) == 0)
    panic("kerneltrap: not from supervisor mode");
  if(intr_get() != 0)
    panic("kerneltrap: interrupts enabled");

  if((which_dev = devintr()) == 0){
    printf("scause %p\n", scause);
    printf("sepc=%p stval=%p\n", r_sepc(), r_stval());
    panic("kerneltrap");
  }

  // give up the CPU if this is a timer interrupt.
  if(which_dev == 2 && myproc() != 0 && myproc()->state == RUNNING)
    yield();

  // the yield() may have caused some traps to occur,
  // so restore trap registers for use by kernelvec.S's sepc instruction.
  w_sepc(sepc);
  w_sstatus(sstatus);
}

// 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)
    ;

  // Tell the C compiler and the processor to not move loads or stores
  // past this point, to ensure that the critical section's memory
  // references happen strictly after the lock is acquired.
  // On RISC-V, this emits a fence instruction.
  __sync_synchronize();

  // Record info about lock acquisition for holding() and debugging.
  lk->cpu = mycpu();
}

void
panic(char *s)
{
  pr.locking = 0;
  printf("panic: ");
  printf(s);
  printf("\n");
  panicked = 1; // freeze uart output from other CPUs
  for(;;)
    ;
}