linux-kernel - Re: [patch V5 28/31] rseq: Switch to fast path processing on exit to user

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Message-ID: <20251023090438.GX3245006@noisy.programming.kicks-ass.net>
Date: Thu, 23 Oct 2025 11:04:38 +0200
From: Peter Zijlstra <peterz@...radead.org>
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>,
	"Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org,
	Sean Christopherson <seanjc@...gle.com>,
	Wei Liu <wei.liu@...nel.org>
Subject: Re: [patch V5 28/31] rseq: Switch to fast path processing on exit to
 user

On Wed, Oct 22, 2025 at 02:52:35PM +0200, Thomas Gleixner wrote:
> +static __always_inline unsigned long __exit_to_user_mode_loop(struct pt_regs *regs,
> +							      unsigned long ti_work)
>  {
>  	/*
>  	 * Before returning to user space ensure that all pending work
> @@ -62,6 +57,23 @@ void __weak arch_do_signal_or_restart(st
>  	return ti_work;
>  }
>  
> +/**
> + * exit_to_user_mode_loop - do any pending work before leaving to user space
> + * @regs:	Pointer to pt_regs on entry stack
> + * @ti_work:	TIF work flags as read by the caller
> + */
> +__always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> +						     unsigned long ti_work)
> +{
> +	for (;;) {
> +		ti_work = __exit_to_user_mode_loop(regs, ti_work);
> +
> +		if (likely(!rseq_exit_to_user_mode_restart(regs)))
> +			return ti_work;
> +		ti_work = read_thread_flags();
> +	}
> +}

Loops in loops seems to be your favourite new theme :-)