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

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Message-ID: <f3390ee6-dfcb-4a97-8aa4-3ba01889c12d@efficios.com>
Date: Thu, 11 Sep 2025 11:42:47 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Jonathan Corbet <corbet@....net>,
 Prakash Sangappa <prakash.sangappa@...cle.com>,
 Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
 K Prateek Nayak <kprateek.nayak@....com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org
Subject: Re: [patch 03/12] rseq: Provide static branch for time slice
 extensions

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@...utronix.de>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> Cc: Boqun Feng <boqun.feng@...il.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@...icios.com>

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


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com