Date: Tue, 8 Aug 2006 13:42:15 -0400 From: John Baldwin <jhb@freebsd.org> To: Suleiman Souhlal <ssouhlal@freebsd.org> Cc: Attilio Rao <attilio@freebsd.org>, freebsd-current@freebsd.org, freebsd-arch@freebsd.org Subject: Re: [PATCH] Adding Solaris-style "owner of records" to rwlocks Message-ID: <200608081342.15839.jhb@freebsd.org> In-Reply-To: <44D7B7ED.5070302@FreeBSD.org> References: <3bbf2fe10608071227j17c4cfa6qd84e1d8e53668fda@mail.gmail.com> <44D7B7ED.5070302@FreeBSD.org>
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On Monday 07 August 2006 18:00, Suleiman Souhlal wrote: > Attilio Rao wrote: > > This is a first implementation of the owner of records concept in rwlocks. > > It allows to avoid the priority inversion problem in the current > > rwlocks implementation (for readers). > > > > The main idea (that John and I discussed) is to have as owner of > > records the first rlock'er for a "class contention". > > The implementation consists in adding two flags (RW_LOCK_OWNED and > > RW_LOCK_EXEMPTED) which are used in order to not penalyze the easy > > case, and syncronizing the operation of acquiring and dropping the > > owner of records with the turnstile spin-lock. > > The main scheme might work in this way: > > > > thread1::rlock() -> sets the owner of records > > thread2::rlock() -> checks for RW_LOCK_OWNED bit and, if it is set, go > > in the easy case > > thread3::rlock() -> checks for RW_LOCK_OWNED... > > thread4::wlock() -> blocks and land its priority to thread1 > > thread1::runlock() -> disable the owner of records (disowning the > > associated turnstile) and sets the RW_LOCK_EXEMPTED flag. In this way > > other threads will treact as an easy case. > > ... > > Aren't you missing the hard part: transferring ownership from one reader > to another? If you don't, you'll still have priority inversions as soon > as the initial reader unlocks.. Even Solaris doesn't do this as the overhead to do this would seem to outweigh the advantages of having a perfect implementation. I think Attilio is actually going to try it several different ways and then run benchmarks to see if that assertion is true. -- John Baldwin
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