Re: [RFC PATCH for 4.18 00/14] Restartable Sequences

[Peter Zijlstra]

On Wed, May 02, 2018 at 08:37:13PM +0000, Daniel Colascione wrote:
> > Recursive locks are teh most horrible crap ever. And having the tid in
> 
> What happened to providing mechanism, not policy?
> 
> You can't wish away recursive locking. It's baked into Java and the CLR,
> and it's enshrined in POSIX. It's not going away, and there's no reason not
> to support it efficiently.

You can implement recursive locks just fine with a TID based word, just
keep the recursion counter external to the futex word. If owner==self,
increment etc..

> > the word allows things like kernel based optimistic spins and possibly
> > PI related things.
> 
> Sure. A lot of people don't want PI though, or at least they want to opt
> into it. And we shouldn't require an entry into the kernel for what we can
> in principle do efficiently in userspace.

Any additional PI would certainly be opt-in, but the kernel based
spinning might make sense unconditionally.