[luau] kicking a threads butt into action

[Bill Luoma]

 Hi Charles.

are you checking for spurious wakeups in your threads?
(using a while loop and a custom flag like: 

pthread_mutex_lock(&t_lock);
 while (!t_cond_flag) {
     err = pthread_cond_wait(&t_cond, &t_lock);
     if (err) { whatever }
}
//do work
t_cond_flag = 0;
pthread_mutex_unlock(&t_lock);

if you're not checking for these "spurious" wakeups,
pthread_cond_wait can return zero... but really not
have gotten a signal. . . in which case it would try
to unlock a mutex it didn't lock. 
which would lead to very undefined behavior.
not sure if this can happen with linux.
the solaris man page warns about it,
as does Stevens.

Bill

----- Original Message ----- 
From: "Charles Lockhart" <lockhart at jeans.ifa.hawaii.edu>
To: "LUAU" <luau at videl.ics.hawaii.edu>
Sent: Sunday, January 05, 2003 8:07 AM
Subject: [luau] kicking a threads butt into action


> I've got an application that consists of three threads:
> 
> Thread 1 services a device on the PCI bus, basically reading data from 
> the device into buffers in memory.  After it's filled a buffer, it calls 
> pthread_cond_signal to start up thread 2, then it goes to sleep for 
> greater than a set period.  This thread seems to be working fine.
> 
> Thread 2, when woken up, confirms that the data in the buffers is ready 
> to process, processes the data into a different set of buffers, calls 
> pthread_cond_signal to wake up thread 3, checks to see if there is any 
> more work to be done, then if not, goes back to sleep.  This thread also 
> seems to be working fine.
> 
> Thread 3, when woken up, writes the data to a file, checks to see if 
> there's any more work to be done, if not goes back to sleep.  This 
> thread is causing me problems.  Every once in a while it just stops 
> working.  I've verified that thread 2's calling of pthread_cond_signal 
> isn't returning an error.  Also, if I throw in printf's for debugging 
> (into thread 3), the problem seems to go away, though for some reason I 
> have this idea that maybe it just starts happening less often, but that 
> could be the paranoia talking.
> 
> So, like, this thread, thread 3, just up and quits doing anything.  If 
> it was just taking a long time to do the stuff, I could probably figure 
> it out, but it just seems to not be doing anything at all.  Do I need to 
> schedule this sucker myself, or what?  Anybody?
> 
> Uh, this is RH7.2, tested running both kernel 2.4.18-3 and 
> 2.4.18-preemptive, the vanilla kernel with the RML preemptive patch.
> 
> Thanks,
> 
> -Charles
> 
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