Do you really "sleep" when blocked on a mutex?

Murty, Ravi ravi.murty at
Mon Apr 21 20:13:14 UTC 2008

Fundamentally it seems that they both come down to inhibiting the thread
and putting them on some queue before calling mi_switch(). But when a
thread is woken up from a sleep, setrunnable is called and it checks to
see if the process is swapped out. No such check is made when a thread
is waiting for a lock (I'm wondering if this is related to how long they
block before becoming runnable which might cause a swapout in one case
and no swapout in the other case?)


-----Original Message-----
From: Julian Elischer [mailto:julian at] 
Sent: Monday, April 21, 2008 12:54 PM
To: Murty, Ravi
Cc: freebsd-hackers at
Subject: Re: Do you really "sleep" when blocked on a mutex?

Murty, Ravi wrote:
> Hello,
> When a thread cannot get a mutex (default mutex) and needs to be
> blocked, is it really put to sleep? From looking at the code it
> that it is inhibited (TD_SET_LOCK) but isn't really put to sleep.
it really has two answers.

1/ sleep has a lot of historical baggage and is expected to work in 
certain ways.

2/ there is a semantic difference between a sleep
(which may sleep for an unbounded time) and being descheduled for
a blocking lock, (Which is supposed to have a guaranteed "shortness"
of duration.

Because sleeps 'may never return' (in the short term) there is a
limit of what you may hold when sleeping. In blocking locks
you may hold other resources, with the expectation that the
other threads will be following the correct locking order and that
the nesting of held resources will be safe, because you will
only be blocked for a moment.

The lowest leven code is the same of course.. things are put on the 
run queue, or not.. Having different higher layers allows us to do
various sanity checks and to enforce the different behaviour.

> 1.	Why isn't it put to sleep - why can't it be treated the same?
> 2.	The eventual question I am trying to answer is the difference
> between setrunnable() and setrunqueue() - this one simply finds a slot
> in the ksegrp and a runq to add the KSE/td. But setrunnable() also
> checks to see if the process is in memory (PS_INMEM) before calling
> sched_wakeup which eventually calls setrunqueue()? Why doesn't
> setrunqueue have to worry about the possibility that the process may
> have been swapped out while it was waiting to become runnable?
> Thanks
> Ravi
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