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Date: Thu, 16 Dec 2021 13:16:07 -0500
From: Rich Felker <dalias@...c.org>
To: zuotina <zuotingyang@....com>
Cc: musl@...ts.openwall.com
Subject: Re: [pthread] pthread_barrier_wait invalid case
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
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