Re: [RFC PATCH 0/1] Add FUTEX_SPIN operation

Peter Zijlstra
2024-04-26T10:15:00Z

On Fri, Apr 26, 2024 at 11:43:51AM +0200, Florian Weimer wrote:
> * André Almeida:
>
> > With FUTEX2_SPIN flag set during a futex_wait(), the futex value is
> > expected to be the PID of the lock owner. Then, the kernel gets the
> > task_struct of the corresponding PID, and checks if it's running. It
> > spins until the futex is awaken, the task is scheduled out or if a
> > timeout happens. If the lock owner is scheduled out at any time, then
> > the syscall follows the normal path of sleeping as usual.
>
> PID or TID?

TID, just like PI_LOCK I would presume.

> I think we'd like to have at least one, possibly more, bits for free
> use, so the kernel ID comparison should at least mask off the MSB,
> possibly more.

Yeah, it should be using FUTEX_TID_MASK -- just like PI_LOCK :-)

I suppose the question is if this thing should then also imply
FUTEX_WAITERS or not.