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From: John Baldwin <jhb@freebsd.org>
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Cc: shrivatsan@gmail.com, Shrivatsan <shrivatsan_v@yahoo.com>
Subject: Re: sched_lock mutex and sleepq_wait
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On Tuesday 23 February 2010 2:34:11 am 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);
>      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?

mi_switch() changes the owner of sched_lock to hand it off to the new thread 
during the switch.  The new thread then returns from mi_switch() and 
eventually returns to something like sleepq_wait() where it unlocks 
sched_lock.

-- 
John Baldwin