sched_lock mutex and sleepq_wait

[Shrivatsan]

Hi,

I am trying to understand how msleep() routine suspends the currently 
executing thread. I see that msleep() calls sleepq_wait().

What I don't understand here is the way in which sched_lock mutex is 
handled.

I took the following snippet from FreeBSD 6:

sleepq_wait(void *wchan)
{
     MPASS(!(curthread->td_flags & TDF_SINTR));
     mtx_lock_spin(&sched_lock);
     sleepq_switch(wchan);
     mtx_unlock_spin(&sched_lock);
}

sched_lock mutex is held, and sleepq_switch() eventually calls 
cpu_switch() which switches to a new thread. 

I don't exactly understand when the sched_lock mutex is released.

Can someone please help me?

Thanks,
-shrivatsan