Re: [RFC PATCH v2 1/4] rseq: Add sched_state field to struct rseq

[Thomas Gleixner]

On Mon, May 29 2023 at 15:14, Mathieu Desnoyers wrote:
> +/*
> + * rseq_sched_state should be aligned on the cache line size.
> + */
> +struct rseq_sched_state {
> +	/*
> +	 * Version of this structure. Populated by the kernel, read by
> +	 * user-space.
> +	 */
> +	__u32 version;
> +	/*
> +	 * The state is updated by the kernel. Read by user-space with
> +	 * single-copy atomicity semantics. This field can be read by any
> +	 * userspace thread. Aligned on 32-bit. Contains a bitmask of enum
> +	 * rseq_sched_state_flags. This field is provided as a hint by the
> +	 * scheduler, and requires that the page holding this state is
> +	 * faulted-in for the state update to be performed by the scheduler.
> +	 */
> +	__u32 state;
> +	/*
> +	 * Thread ID associated with the thread registering this structure.
> +	 * Initialized by user-space before registration.
> +	 */
> +	__u32 tid;
> +};
> +
>  /*
>   * struct rseq is aligned on 4 * 8 bytes to ensure it is always
>   * contained within a single cache-line.
> @@ -148,6 +180,15 @@ struct rseq {
>  	 */
>  	__u32 mm_cid;
>  
> +	__u32 padding1;
> +
> +	/*
> +	 * Restartable sequences sched_state_ptr field. Initialized by
> +	 * userspace to the address at which the struct rseq_sched_state is
> +	 * located. Read by the kernel on rseq registration.
> +	 */
> +	__u64 sched_state_ptr;
> +

Why is this a separate entity instead of being simply embedded into
struct rseq?

Neither the code comment nor the changelog tells anything about that.

If your intention was to provide a solution for process shared futexes
then you completely failed to understand how process shared futexes
work. If not, then please explain why you need a pointer and the
associated hackery to deal with it.

Thanks,

        tglx