Re: [PATCH 05/10] entry: Split preemption from irqentry_exit_to_kernel_mode()

[Jinjie Ruan]

On 2026/4/7 21:16, Mark Rutland wrote:
> Some architecture-specific work needs to be performed between the state
> management for exception entry/exit and the "real" work to handle the
> exception. For example, arm64 needs to manipulate a number of exception
> masking bits, with different exceptions requiring different masking.
> 
> Generally this can all be hidden in the architecture code, but for arm64
> the current structure of irqentry_exit_to_kernel_mode() makes this
> particularly difficult to handle in a way that is correct, maintainable,
> and efficient.
> 
> The gory details are described in the thread surrounding:
> 
>   https://lore.kernel.org/lkml/acPAzdtjK5w-rNqC@J2N7QTR9R3/
> 
> The summary is:
> 
> * Currently, irqentry_exit_to_kernel_mode() handles both involuntary
>   preemption AND state management necessary for exception return.
> 
> * When scheduling (including involuntary preemption), arm64 needs to
>   have all arm64-specific exceptions unmasked, though regular interrupts
>   must be masked.
> 
> * Prior to the state management for exception return, arm64 needs to
>   mask a number of arm64-specific exceptions, and perform some work with
>   these exceptions masked (with RCU watching, etc).
> 
> While in theory it is possible to handle this with a new arch_*() hook
> called somewhere under irqentry_exit_to_kernel_mode(), this is fragile
> and complicated, and doesn't match the flow used for exception return to
> user mode, which has a separate 'prepare' step (where preemption can
> occur) prior to the state management.
> 
> To solve this, refactor irqentry_exit_to_kernel_mode() to match the
> style of {irqentry,syscall}_exit_to_user_mode(), moving preemption logic
> into a new irqentry_exit_to_kernel_mode_preempt() function, and moving
> state management in a new irqentry_exit_to_kernel_mode_after_preempt()
> function. The existing irqentry_exit_to_kernel_mode() is left as a
> caller of both of these, avoiding the need to modify existing callers.
> 
> There should be no functional change as a result of this patch.
> 
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxx>
> Cc: Vladimir Murzin <vladimir.murzin@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> ---
>  include/linux/irq-entry-common.h | 26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)
> 
> Thomas/Peter/Andy, as mentioned on IRC, I haven't created kerneldoc
> comments for these new functions because the existing comments don't
> seem all that consistent (e.g. for user mode vs kernel mode), and I
> suspect we want to rewrite them all in one go for wider consistency.
> 
> I'm happy to respin this, or to follow-up with that as per your
> preference.
> 
> Mark.
> 
> diff --git a/include/linux/irq-entry-common.h b/include/linux/irq-entry-common.h
> index 2206150e526d8..24830baa539c6 100644
> --- a/include/linux/irq-entry-common.h
> +++ b/include/linux/irq-entry-common.h
> @@ -421,10 +421,18 @@ static __always_inline irqentry_state_t irqentry_enter_from_kernel_mode(struct p
>  	return ret;
>  }
>  
> -static __always_inline void irqentry_exit_to_kernel_mode(struct pt_regs *regs, irqentry_state_t state)
> +static inline void irqentry_exit_to_kernel_mode_preempt(struct pt_regs *regs, irqentry_state_t state)
>  {
> -	lockdep_assert_irqs_disabled();
> +	if (regs_irqs_disabled(regs) || state.exit_rcu)
> +		return;
> +
> +	if (IS_ENABLED(CONFIG_PREEMPTION))
> +		irqentry_exit_cond_resched();
> +}
>  
> +static __always_inline void
> +irqentry_exit_to_kernel_mode_after_preempt(struct pt_regs *regs, irqentry_state_t state)
> +{
>  	if (!regs_irqs_disabled(regs)) {
>  		/*
>  		 * If RCU was not watching on entry this needs to be done
> @@ -443,9 +451,6 @@ static __always_inline void irqentry_exit_to_kernel_mode(struct pt_regs *regs, i
>  		}
>  
>  		instrumentation_begin();
> -		if (IS_ENABLED(CONFIG_PREEMPTION))
> -			irqentry_exit_cond_resched();
> -
>  		/* Covers both tracing and lockdep */
>  		trace_hardirqs_on();
>  		instrumentation_end();
> @@ -459,6 +464,17 @@ static __always_inline void irqentry_exit_to_kernel_mode(struct pt_regs *regs, i
>  	}
>  }
>  
> +static __always_inline void irqentry_exit_to_kernel_mode(struct pt_regs *regs, irqentry_state_t state)
> +{
> +	lockdep_assert_irqs_disabled();
> +
> +	instrumentation_begin();
> +	irqentry_exit_to_kernel_mode_preempt(regs, state);
> +	instrumentation_end();
> +
> +	irqentry_exit_to_kernel_mode_after_preempt(regs, state);
> +}

Reviewed-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>

> +
>  /**
>   * irqentry_enter - Handle state tracking on ordinary interrupt entries
>   * @regs:	Pointer to pt_regs of interrupted context