linux-kernel - Re: [PATCH -next v4 07/19] arm64: entry: Call arm64_preempt_schedule

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Message-ID: <ZyD3TiU1Dk_zYsXE@J2N7QTR9R3.cambridge.arm.com>
Date: Tue, 29 Oct 2024 14:55:10 +0000
From: Mark Rutland <mark.rutland@....com>
To: Jinjie Ruan <ruanjinjie@...wei.com>
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Subject: Re: [PATCH -next v4 07/19] arm64: entry: Call
 arm64_preempt_schedule_irq() only if irqs enabled

On Fri, Oct 25, 2024 at 06:06:48PM +0800, Jinjie Ruan wrote:
> Only if irqs are enabled when the interrupt trapped, there may be
> a chance to reschedule after the interrupt has been handled, so move
> arm64_preempt_schedule_irq() into regs_irqs_disabled() check false
> if block.
> 
> As Mark pointed out, this change will have the following key impact:
> 
>     "We will not preempt when taking interrupts from a region of kernel
>     code where IRQs are enabled but RCU is not watching, matching the
>     behaviour of the generic entry code.
> 
>     This has the potential to introduce livelock if we can ever have a
>     screaming interrupt in such a region, so we'll need to go figure out
>     whether that's actually a problem.
> 
>     Having this as a separate patch will make it easier to test/bisect
>     for that specifically."
> 
> Suggested-by: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Jinjie Ruan <ruanjinjie@...wei.com>

This should be folded into the prior patch.

Mark.

> ---
>  arch/arm64/kernel/entry-common.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index e0380812d71e..b57f6dc66115 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -114,8 +114,6 @@ static void __sched arm64_preempt_schedule_irq(void)
>  static void noinstr exit_to_kernel_mode(struct pt_regs *regs,
>  					irqentry_state_t state)
>  {
> -	arm64_preempt_schedule_irq();
> -
>  	mte_check_tfsr_exit();
>  
>  	lockdep_assert_irqs_disabled();
> @@ -129,6 +127,8 @@ static void noinstr exit_to_kernel_mode(struct pt_regs *regs,
>  			return;
>  		}
>  
> +		arm64_preempt_schedule_irq();
> +
>  		trace_hardirqs_on();
>  	} else {
>  		if (state.exit_rcu)
> -- 
> 2.34.1
>