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

2006/8/8, Suleiman Souhlal <ssouhlal@xxxxxxxxxxx>:
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..

Exactly, but having a complete owner switching would be:
1) too hard to achieve in terms of resource taken
2) will imply too many races and we might get a too hard function

With this implementation, only the first rlock (for every class
contention) will be penalyzed while the other are treacted as the
easy/hard case.
It doesn't completely solve the priority inversion problem, but it's
the better compromise between performances/correctnes.

Attilio


--
Peace can only be achieved by understanding - A. Einstein
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