linux-kernel - Re: [patch V5 25/31] rseq: Rework the TIF

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Message-ID: <aPjcmhnOm1R-9euo@google.com>
Date: Wed, 22 Oct 2025 06:31:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>, Michael Jeanson <mjeanson@...icios.com>, 
	Jens Axboe <axboe@...nel.dk>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Peter Zijlstra <peterz@...radead.org>, "Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org, 
	Wei Liu <wei.liu@...nel.org>
Subject: Re: [patch V5 25/31] rseq: Rework the TIF_NOTIFY handler

On Wed, Oct 22, 2025, Thomas Gleixner wrote:
> ---
>  include/linux/rseq_entry.h |   29 +++++++++++++++++
>  kernel/rseq.c              |   76 +++++++++++++++++++--------------------------
>  2 files changed, 62 insertions(+), 43 deletions(-)
> --- a/include/linux/rseq_entry.h
> +++ b/include/linux/rseq_entry.h
> @@ -368,6 +368,35 @@ bool rseq_set_ids_get_csaddr(struct task
>  	return false;
>  }
>  
> +/*
> + * Update user space with new IDs and conditionally check whether the task
> + * is in a critical section.
> + */
> +static rseq_inline bool rseq_update_usr(struct task_struct *t, struct pt_regs *regs,
> +					struct rseq_ids *ids, u32 node_id)
> +{
> +	u64 csaddr;
> +
> +	if (!rseq_set_ids_get_csaddr(t, ids, node_id, &csaddr))
> +		return false;
> +
> +	/*
> +	 * On architectures which utilize the generic entry code this
> +	 * allows to skip the critical section when the entry was not from
> +	 * a user space interrupt, unless debug mode is enabled.
> +	 */
> +	if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY)) {
> +		if (!static_branch_unlikely(&rseq_debug_enabled)) {
> +			if (likely(!t->rseq.event.user_irq))
> +				return true;
> +		}
> +	}

This can be:

	if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY) &&
	    !static_branch_unlikely(&rseq_debug_enabled) &&
	    likely(!t->rseq.event.user_irq))
		return true;

> +	if (likely(!csaddr))
> +		return true;
> +	/* Sigh, this really needs to do work */
> +	return rseq_update_user_cs(t, regs, csaddr);
> +}
> +
>  static __always_inline void rseq_exit_to_user_mode(void)
>  {
>  	struct rseq_event *ev = &current->rseq.event;