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

[Daniel Colascione]

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.

> 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?