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Date: Fri, 17 Dec 2021 22:28:14 +0800 (CST)
From: zuotina <zuotingyang@....com>
To: musl@...ts.openwall.com
Subject: Re:Re: [pthread] pthread_barrier_wait invalid case
At 2021-12-17 02:16:07, "Rich Felker" <dalias@...c.org> wrote:
>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?
running on aarch64.
Looking forward to fix, thank you
>> 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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