[musl] Re:[musl] Re:Re: [musl] [pthread] pthread_barrier_wait invalid case

[zuotina <zuotingyang@126.com>]

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Hi Team,
Simple feedback on this issue
First,  replace pthread_barrier_wait in timer_create with a custom sync function (implemented by __wait, __wake),
then the problem of panic is solved
But I still think the best way is fixing pthread_barrier_wait. 


In addition, it is also the problem of the timer_create function. Continue to ask for advice.
```c
timer_create:
case SIGEV_THREAD:
r = pthread_create(&td, &attr, start, &args);
    ...
if (syscall(SYS_timer_create, clk, &ksev, &timerid) < 0)
timerid = -1;
```
If this syscall fails, the 'start' thread will reside permanently, 
so the above only sets timerid = -1, which should not be perfect ?
```c
start:
for (;;) {
while (sigwaitinfo(SIGTIMER_SET, &si) < 0);
}

```







At 2021-12-17 22:28:14, "zuotina" <zuotingyang@126.com> wrote:

At 2021-12-17 02:16:07, "Rich Felker" <dalias@libc.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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