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From: zuotina <zuotingyang@126.com>
To: musl@lists.openwall.com
Subject: [musl] [pthread] pthread_barrier_wait  invalid case
Date: Thu, 16 Dec 2021 23:25:35 +0800 (CST)	[thread overview]
Message-ID: <57cc3430.7783.17dc3d9431a.Coremail.zuotingyang@126.com> (raw)

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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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             reply	other threads:[~2021-12-16 15:25 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-16 15:25 zuotina [this message]
2021-12-16 18:16 ` Rich Felker
2021-12-17 14:28   ` [musl] " zuotina
2022-01-19 14:56     ` [musl] " zuotina
2022-01-20  2:19       ` 答复: " zhaohang (F)

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