sched_lock mutex and sleepq_wait

[Julian Elischer]

Shrivatsan wrote:
> 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);
----we sleep here-----
       some other thread runs, When it wakes up it magicall owns the
       schedlock which it releases. then it sleeps and locks schedlock
       again.A
----- we get run.. magically the schedlock is changed so that we
       own it.  now we return, maybe 5 minutes later and must free it.

>      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
> 
> 
>       
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