need for another mutex type/flag?

Kip Macy kmacy at
Sat Jan 24 20:25:42 PST 2009

The adaptive spinning of regular mutexes already satisfies your need
for "short" hold. You might wish to add a thread flag used when
INVARIANTS is enabled that is set when a leaf mutex is acquired and
checked on all mutex acquisitions.


On Sat, Jan 24, 2009 at 3:49 PM, Julian Elischer <julian at> wrote:
> Currently we have:
> spin locks..
> you really don't want to hold these for
> any time at all, and this is enforced to some extent in the waiter.
> regular mutexes..
> You can hold these for as long as you want but teh shorter
> the better and you can't sleep when holding them. The
> "shortness" of the time of holding the mutex is not enforced.
> "Sleeps" (including sx-locks and friends)
>  You may hold these or be descheduled for really long periods of time.
> Now it occurs to me that there is a subclass of regular mutexes,
> usage, which is where you want to use a mutex to guard some small
> but critical structure, and that you know that access to that structure will
> be quick, and that you can guarantee that you will
> not acquire any other locks (which could introduce unknown delay)
> while hoding the lock.
> One way of thinking about this is that this lock would always be
> a leaf node on the tree of lock orders.
> I would like to be able to add a flag to a mutex
> that tags it as a 'leaf' mutex. As a result it would be illegal
> to take any other mutex while holding a leaf mutex. Somewhat
> similar to the way that it is illegal to take aregular
> mutex while holding a spin mutex..
> In netgraph I have a stipulation that is hard to specify which
> is that  you MAY take a mutex in a netgraph node if you can guarantee
> that the mutex WILL be satisfied very quickly, but it'd
> be nice to be able to specify "you may only take 'leaf' mutexes within an
> netgraph node".
> thoughts? (especially from jhb and other locking types).
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