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

André Almeida
2024-05-01T23:45:13Z

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?