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