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

[Christian Brauner]

On Wed, May 01, 2024 at 08:44:36PM -0300, André Almeida wrote:
> Hi Christian,
> 
> Em 26/04/2024 07:26, Christian Brauner escreveu:
> > On Thu, Apr 25, 2024 at 05:43:31PM -0300, André Almeida wrote:
> > > Hi,
> > > 
> > > In the last LPC, Mathieu Desnoyers and I presented[0] a proposal to extend the
> > > rseq interface to be able to implement spin locks in userspace correctly. Thomas
> > > Gleixner agreed that this is something that Linux could improve, but asked for
> > > an alternative proposal first: a futex operation that allows to spin a user
> > > lock inside the kernel. This patchset implements a prototype of this idea for
> > > further discussion.
> > > 
> > > 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.
> > > 
> > > If the futex is awaken and we are spinning, we can return to userspace quickly,
> > > avoid the scheduling out and in again to wake from a futex_wait(), thus
> > > speeding up the wait operation.
> > > 
> > > I didn't manage to find a good mechanism to prevent race conditions between
> > > setting *futex = PID in userspace and doing find_get_task_by_vpid(PID) in kernel
> > > space, giving that there's enough room for the original PID owner exit and such
> > > PID to be relocated to another unrelated task in the system. I didn't performed
> > 
> > One option would be to also allow pidfds. Starting with v6.9 they can be
> > used to reference individual threads.
> > 
> > So for the really fast case where you have multiple threads and you
> > somehow may really do care about the impact of the atomic_long_inc() on
> > pidfd_file->f_count during fdget() (for the single-threaded case the
> > increment is elided), callers can pass the TID. But in cases where the
> > inc and put aren't a performance sensitive, you can use pidfds.
> > 
> 
> Thank you very much for making the effort here, much appreciated :)
> 
> While I agree that pidfds would fix the PID race conditions, I will move
> this interface to support TIDs instead, as noted by Florian and Peter. With
> TID the race conditions are diminished I reckon?

Unless I'm missing something the question here is PID (as in TGID aka
thread-group leader id gotten via getpid()) vs TID (thread specific id
gotten via gettid()). You want the thread-specific id as you want to
interact with the futex state of a specific thread not the thread-group
leader.

Aside from that TIDs are subject to the same race conditions that PIDs
are. They are allocated from the same pool (see alloc_pid()).