Re: [rfc] "fair" rw spinlocks

Linus Torvalds
2009-11-28T17:30:23Z

On Mon, 23 Nov 2009, Nick Piggin wrote:
>
> Last time this issue came up that I could see, I don't think
> there were objections to making rwlocks fair, the main
> difficulty seemed to be that we allow reentrant read locks
> (so a write lock waiting must not block arbitrary read lockers).

We have at least one major rwlock user - tasklist_lock or whatever. And
that one definitely depends on being able to do 'rwlock()' in an
interrupt, without other rwlock'ers having to disable irq's (even if there
might be a new writer coming in on another cpu).

That usage case _might_ be turned into RCU or something similar, in which
case I don't think any major rwlock users remain. However, if that's the
case, then why should anybody care about fairness any more either?

So as far as I can tell, we have only one real user of rwlocks where
livelocks might be relevant, but that one real user absolutely _requires_
the unfair behavior.

Linus