Re: [musl] [pthread] pthread_barrier_wait invalid case

[Rich Felker <dalias@libc.org>]

On Thu, Dec 16, 2021 at 11:25:35PM +0800, zuotina wrote:
> 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);
> ```

The intent here is that it's not possible that b has been released,
because all waiters have to synchronize on b->_b_inst. It's possible
there's a bug here. I'll look. What arch are you running on?

> 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. 

Thanks for the report.

Rich