Re: [patch 03/12] rseq: Provide static branch for time slice extensions

Mathieu Desnoyers
2025-09-11T15:42:57Z

On 2025-09-08 18:59, Thomas Gleixner wrote:
> Guard the time slice extension functionality with a static key, which can
> be disabled on the kernel command line.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> ---
> include/linux/rseq_entry.h | 11 +++++++++++
> kernel/rseq.c | 17 +++++++++++++++++
> 2 files changed, 28 insertions(+)
>
> --- a/include/linux/rseq_entry.h
> +++ b/include/linux/rseq_entry.h
> @@ -77,6 +77,17 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEB
> #define rseq_inline __always_inline
> #endif
>
> +#ifdef CONFIG_RSEQ_SLICE_EXTENSION
> +DECLARE_STATIC_KEY_TRUE(rseq_slice_extension_key);
> +
> +static __always_inline bool rseq_slice_extension_enabled(void)
> +{
> + return static_branch_likely(&rseq_slice_extension_key);
> +}
> +#else /* CONFIG_RSEQ_SLICE_EXTENSION */
> +static inline bool rseq_slice_extension_enabled(void) { return false; }
> +#endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
> +
> bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr);
> bool rseq_debug_validate_ids(struct task_struct *t);
>
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -474,3 +474,20 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
>
> return 0;
> }
> +
> +#ifdef CONFIG_RSEQ_SLICE_EXTENSION
> +DEFINE_STATIC_KEY_TRUE(rseq_slice_extension_key);
> +
> +static int __init rseq_slice_cmdline(char *str)
> +{
> + bool on;
> +
> + if (kstrtobool(str, &on))
> + return -EINVAL;
> +
> + if (!on)
> + static_branch_disable(&rseq_slice_extension_key);
> + return 0;

as pointed out elsewhere, this should be return 1.

Other than that:

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

> +}
> +__setup("rseq_slice_ext=", rseq_slice_cmdline);
> +#endif /* CONFIG_RSEQ_SLICE_EXTENSION */
>

--
Mathieu Desnoyers
EfficiOS Inc.