ubuntu: sem_timedwait not waking (C)

Question

I have 3 processes which need to be synchronized. Process one does something then wakes process two and sleeps, which does something then wakes process three and sleeps, which d

Answer 1

I have no clue on what is going wrong and the code looks fine too me. Here are some things you could do do somehow get more information.

• use a different timeout, both shorter and longer, and see if your problem still occurs.
• use a non timed version, and see if the programm hangs.
• try to modify the behaviour of your kernel scheduler, for example using kernel command line parameters, or using procfs or sysfs.

As pointed by Jens, there are two races :

The first is when evaluationg the value of the semaphore, after the call to sem_timedwait. This is not changing the control flow whit respects to semaphore. Wether the thread timedout or not, it still goes through the "should I trigger the next thread" block.

The second is in the "Should I wakeup the next thread" part. We could have the following events :

1. Threads n calls sem_getvalue(trigger) and gets a 1
2. Thread n+1 returns from sem_timedwait and the semaphore goes to 0
3. Thread n decides not to post and the semaphore stays to 0

Now, I can't see how this could trigger the observed behaviour. After all, since Thread n+1 is waked up anyway, it will in turn wake up thread n+2 which will wake up thread n etc...

While it is possible to get glitches, I can't see how this could lead to systematic timeout from a thread.