[PATCH] Adding Solaris-style "owner of records" to rwlocks

John Baldwin jhb at freebsd.org
Tue Aug 8 18:02:06 UTC 2006

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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