need for another mutex type/flag?

[Jeff Roberson]

On Sun, 25 Jan 2009, Julian Elischer wrote:

> Kip Macy wrote:
>> 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.
>
> ummm that was what I was asking for.. an official designation of a 'leaf' 
> mutex...

It'd be much easier to add the flag as a mtx_init() flag and have witness 
check it.  It'd only take 15 minutes to do properly.

What is the case you have where you can not acquire non leaf mutexes?

Thanks,
Jeff

>
>> 
>> -Kip
>> 
>> On Sat, Jan 24, 2009 at 3:49 PM, Julian Elischer <julian at elischer.org> 
>> 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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>
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