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

Suleiman Souhlal ssouhlal at FreeBSD.org
Mon Aug 7 22:00:34 UTC 2006

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..

-- Suleiman

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