How to avoid zombie kernel threads?

Jarno Paananen (jpaana@s2.org)
30 Jan 2002 06:06:50 +0200


Hi,

I'm coding a driver (can be found from
http://hardsid.sourceforge.net/ is someone is actually interested)
that uses a kernel thread to do the actual work asynchronously from
rest of the world. The thread is created when opening a character
device and exits when the device is closed.

This works fine otherwise but if the user mode process opens and
closes the device multiple times during its lifetime I get N-1
zombie kernel threads where N is the number of opens.

The code goes like this:

in device open:

DECLARE_MUTEX_LOCKED(sem);
int rmmod = 0;

rmmod = 0;
notify = &sem;
kernel_thread(hsid_thread, (void *)sid_data, 0);
down(&sem);
notify = NULL;

in device close:

notify = &sem;
rmmod = 1;
up(&todoSem); // just to wake the thread to do something
down(&sem);
notify = NULL;
rmmod = 0;

and the thread itself does:

[daemonize() etc.]

/* Notify the parent */
if(notify != NULL)
up(notify);

for(;;)
{
if (rmmod || signal_pending(current))
break;

/* We sit here waiting for something to do */
down_interruptible(&todoSem);

if (rmmod || signal_pending(current))
break;

[actual work]
}

if(notify != NULL)
up(notify);

return 0;

I think this worked fine in earlier 2.4 versions (not sure though),
but I'm now seeing this in both 2.4.18-pre7 and 2.5.2-dj6, UP and SMP.

Thanks,

// Jarno
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