Re: [patch v2 08/11] futex: Add robust futex unlock IP range

[Sebastian Andrzej Siewior]

On 2026-03-20 00:24:46 [+0100], Thomas Gleixner wrote:
> --- a/include/linux/futex_types.h
> +++ b/include/linux/futex_types.h
> @@ -31,6 +31,20 @@ struct futex_sched_data {
…
>  
> +struct futex_unlock_cs_range {
> +	unsigned long	       start_ip;
> +	unsigned long	       end_ip;
> +	unsigned int	       pop_size32;
> +};
> +
> +#define FUTEX_ROBUST_MAX_CS_RANGES	2
…
> @@ -50,6 +68,10 @@ struct futex_mm_data {
>  	atomic_long_t			phash_atomic;
>  	unsigned int			__percpu *phash_ref;
>  #endif
> +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK
> +	unsigned int			unlock_cs_num_ranges;
> +	struct futex_unlock_cs_range	unlock_cs_ranges[FUTEX_ROBUST_MAX_CS_RANGES];
> +#endif
>  };

While looking at this from an economic point of view, we get:
|        unsigned int *             phash_ref;            /*    80     8 */
|        unsigned int               unlock_cs_num_ranges; /*    88     4 */
|
|        /* XXX 4 bytes hole, try to pack */
|
|        struct futex_unlock_cs_range unlock_cs_ranges[2]; /*    96    48 */
|}
|struct futex_unlock_cs_range {
|        long unsigned int          start_ip;             /*     0     8 */
|        long unsigned int          end_ip;               /*     8     8 */
|        unsigned int               pop_size32;           /*    16     4 */
|
|        /* size: 24, cachelines: 1, members: 3 */
|        /* padding: 4 */
|        /* last cacheline: 24 bytes */
|};

end_ip could be replaced with a u16 size. There is no need to have
pop_size32 as u32, it could be a u16 filling the gap.
On the other hand, pop_size32 could be passed by the caller since it is
known if it is the first or the second member / the 64bit or 32bit case.

unlock_cs_num_ranges could probably go because if start_ip == NULL then
there is no mapping since it can't be mapped at 0x0. Worst case would be
to check two variables vs NULL.

And if we replace end_ip with size then we could remove it because vdso
is known at compile so we should know the size at compile time.

Sebastian