mailing list of musl libc
 help / color / mirror / code / Atom feed
* [musl] [pthread] pthread_barrier_wait  invalid case
@ 2021-12-16 15:25 zuotina
  2021-12-16 18:16 ` Rich Felker
  0 siblings, 1 reply; 5+ messages in thread
From: zuotina @ 2021-12-16 15:25 UTC (permalink / raw)
  To: musl

[-- Attachment #1: Type: text/plain, Size: 1727 bytes --]

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. 

[-- Attachment #2: Type: text/html, Size: 2764 bytes --]

^ permalink raw reply	[flat|nested] 5+ messages in thread

end of thread, other threads:[~2022-01-20  2:20 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-12-16 15:25 [musl] [pthread] pthread_barrier_wait invalid case zuotina
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)

Code repositories for project(s) associated with this inbox:

	https://git.vuxu.org/mirror/musl/

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).