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

[Peter Zijlstra]

On Wed, May 02, 2018 at 06:27:22PM +0000, Daniel Colascione wrote:
> On Wed, May 2, 2018 at 10:22 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >> On Wed, May 02, 2018 at 03:53:47AM +0000, Daniel Colascione wrote:
> > > Suppose we make a userspace mutex implemented with a lock word having
> three
> > > bits: acquired, sleep_mode, and wait_pending, with the rest of the word
> not
> > > being relevant at the moment.
> 
> > So ideally we'd kill FUTEX_WAIT/FUTEX_WAKE for mutexes entirely, and go
> > with FUTEX_LOCK/FUTEX_UNLOCK that have the same semantics as the
> > existing FUTEX_LOCK_PI/FUTEX_UNLOCK_PI, namely, the word contains the
> > owner TID.
> 
> That doesn't work if you want to use the rest of the word for something
> else, like a recursion count. With FUTEX_WAIT and FUTEX_WAKE, you can make
> a lock with two bits.

Recursive locks are teh most horrible crap ever. And having the tid in
the word allows things like kernel based optimistic spins and possibly
PI related things.

> > As brought up in the last time we talked about spin loops, why do we
> > care if the spin loop is in userspace or not? Aside from the whole PTI
> > thing, the syscall cost was around 150 cycle or so, while a LOCK CMPXCHG
> > is around 20 cycles. So ~7 spins gets you the cost of entry.
> 
> That's pre-KPTI, isn't it?

Yes, and once the hardware gets sorted, we'll be there again. I don't
think we should design interfaces for 'broken' hardware.