Date: Sun, 25 Jan 2009 09:11:53 +0000 From: Kip Macy <kmacy@freebsd.org> To: Julian Elischer <julian@elischer.org> Cc: arch@freebsd.org Subject: Re: need for another mutex type/flag? Message-ID: <3c1674c90901250111w34f9d016xb8e812cf46433970@mail.gmail.com> In-Reply-To: <497C235E.5090807@elischer.org> References: <497BA91D.805@elischer.org> <3c1674c90901241956j244ed067p7ff4df5454beba82@mail.gmail.com> <497C235E.5090807@elischer.org>
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On Sun, Jan 25, 2009 at 8:31 AM, Julian Elischer <julian@elischer.org> 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... uhm ... I was explaining a way to implement it without WITNESS. -Kip > >> >> -Kip >> >> On Sat, Jan 24, 2009 at 3:49 PM, Julian Elischer <julian@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). >>> >>> >>> _______________________________________________ >>> freebsd-arch@freebsd.org mailing list >>> http://lists.freebsd.org/mailman/listinfo/freebsd-arch >>> To unsubscribe, send any mail to "freebsd-arch-unsubscribe@freebsd.org" >>> > >
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