[musl] [pthread] pthread_barrier_wait invalid case

[zuotina <zuotingyang@126.com>]

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Hi everrone


I encountered a panic problem when using timer_create recently.
Although the probability is small, it still happened.
Finaly I found there is a problem in the code of phtread_barrier_wait, 
and review code found that there may be problems in the following place, 
81  a_store(&b->_b_lock, 0);
82  if (b->_b_waiters) __wake(&b->_b_lock, 1, 1);
If scheduling occurs between lines 81 and 82, it will be not good.
So I did an experiment and modified the source code of pthread_barrier_wait to verify my guess
```c
81  a_store(&b->_b_lock, 0);
                 /* If it is scheduled out here, when another thread executes pthread_barrier_wait again, 
                    it can go through the entire function happily, that is, it will not be blocked */
      syscall(yiled); // new add for test
               // When the dispatch comes back, this b has been released
82  if (b->_b_waiters) __wake(&b->_b_lock, 1, 1);
```
Here is an example of timer_create (src/time/timer_create.c)
There are two threads A and B call pthread_barrier_wait. 
The call is as follows
A thread: (timer_create // parent thread)
{
       .....
      // new add for test---begin
       while(b->_b_inst == NULL) {
                syscall(yield);
       }
     // new add for test---end
     pthread_barrier_wait();
}
B thread: (start // child thread)
{
       .....
      //  Ensure that this function is advanced to the if (!inst) {} branch of barrier_wait
      pthread_barrier_wait();
}


In short, the reason for panic is that pthread_barrier_wait is not blocked as expected;
I hope you help to confirm whether there is a problem with the implementation 
of pthread_barrier_wait or am I wrong?


Looking forward to your reply. Thank you. 

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