Re: Pthread-win32 races?

[Ross Johnson <Ross.Johnson@homemail.com.au>]

Sergey Fokin wrote:
> Hi all.
>
> I have some peculiarities with pthread-win32 and suppose there's a bug
> in library. 
Sergey,

Although I haven't tested the fix yet, it is possible in the current 
version for sem_destroy to incorrectly return EBUSY. The problem is in 
sem_destroy.c. In the sample case, sem_post releases the waiting thread 
which can then enter sem_destroy while sem_post still holds the mutex 
guarding the semaphore's state. The EBUSY error results from 
sem_destroy's "trylock" attempt to acquire that mutex. It should block 
instead.

I'm about to test a fix for this, which if successful will also remove 
another race that occurs around invalidating the destroyed semaphore.

Ross
>
> Here's my example code:
>
> #include <pthread.h>
> #include <semaphore.h>
> #include <stdio.h>
> #include <assert.h>
>
> void * thr(void * arg)
> {
>  sem_post((sem_t*)arg);
>  return 0;
> }
>
> int main()
> {
>    sem_t sem;
>    int error = 0;
>    error = sem_init(&sem, 0, 0); // OK
>    assert(!error);
>
>    pthread_t thread;
>    error = pthread_create(&thread, 0, thr, &sem); // OK
>    assert(!error);
>
>    sem_wait(&sem);
>    error = sem_destroy(&sem);
>    if (error != 0)
>    {
>        error = errno; // errno == 16 (EBUSY)
>        printf("errno = %d\n", error);
>    }
>
>    pthread_join(thread, 0);
>
>    return error;
> }
>
>
> So, here we have error 16 (0x10) in almost all runs, independently on
> compile options (at least i couldn't find working combination).
> Compiler is msvc 7.1 sp1.
>
> I've read about some troubles with it in BUGS file, but first, this
> one is unrelated to those, as I can see, second all tests from `tests'
> directory run with no errors, but this one fails even with same
> compile options.
>
> When I've tried to debug, it turned out that when main() is executing
> sem_destroy(), child thread is still in sem_post(). But I couldn't
> find out what's going on there and supposed this is some kind of race,
> this is why subject is about races.
>
> Google didn't find similar issues about pthread-win32 library.
>
> Really hope, this is my fault, but I have no idea where I'm wrong.
>
> Thanks in advance.
>
> PS I'll also try gcc for win32 and msvc8.0 later - I don't have them
> on this computer.
>