linux-kernel - Re: [PATCH v2] riscv: Enable interrupt during exception handling

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Message-ID: <6a07cf54-8e97-4726-ba38-67feb90f3e7f@ghiti.fr>
Date: Wed, 9 Jul 2025 09:00:58 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Nam Cao <namcao@...utronix.de>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Peter Zijlstra <peterz@...radead.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
 linux-rt-devel@...ts.linux.dev, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] riscv: Enable interrupt during exception handling

Hi Nam,

On 6/25/25 10:56, Nam Cao wrote:
> force_sig_fault() takes a spinlock, which is a sleeping lock with
> CONFIG_PREEMPT_RT=y. However, exception handling calls force_sig_fault()
> with interrupt disabled, causing a sleeping in atomic context warning.
>
> This can be reproduced using userspace programs such as:
>      int main() { asm ("ebreak"); }
> or
>      int main() { asm ("unimp"); }
>
> There is no reason that interrupt must be disabled while handling
> exceptions from userspace.
>
> Enable interrupt while handling user exceptions. This also has the added
> benefit of avoiding unnecessary delays in interrupt handling.
>
> Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Signed-off-by: Nam Cao <namcao@...utronix.de>
> ---
> v2: stop enabling interrupts for kernel exceptions. For exceptions treated
> like NMI, it is wrong. For page faults, interrupts are already
> (conditionally) enabled.
> ---
>   arch/riscv/kernel/traps.c | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 9c83848797a78..80230de167def 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -6,6 +6,7 @@
>   #include <linux/cpu.h>
>   #include <linux/kernel.h>
>   #include <linux/init.h>
> +#include <linux/irqflags.h>
>   #include <linux/randomize_kstack.h>
>   #include <linux/sched.h>
>   #include <linux/sched/debug.h>
> @@ -151,7 +152,9 @@ asmlinkage __visible __trap_section void name(struct pt_regs *regs)		\
>   {										\
>   	if (user_mode(regs)) {							\
>   		irqentry_enter_from_user_mode(regs);				\
> +		local_irq_enable();						\
>   		do_trap_error(regs, signo, code, regs->epc, "Oops - " str);	\
> +		local_irq_disable();						\
>   		irqentry_exit_to_user_mode(regs);				\
>   	} else {								\
>   		irqentry_state_t state = irqentry_nmi_enter(regs);		\
> @@ -173,17 +176,14 @@ asmlinkage __visible __trap_section void do_trap_insn_illegal(struct pt_regs *re
>   
>   	if (user_mode(regs)) {
>   		irqentry_enter_from_user_mode(regs);
> -
>   		local_irq_enable();
>   
>   		handled = riscv_v_first_use_handler(regs);
> -
> -		local_irq_disable();
> -
>   		if (!handled)
>   			do_trap_error(regs, SIGILL, ILL_ILLOPC, regs->epc,
>   				      "Oops - illegal instruction");
>   
> +		local_irq_disable();
>   		irqentry_exit_to_user_mode(regs);
>   	} else {
>   		irqentry_state_t state = irqentry_nmi_enter(regs);
> @@ -308,9 +308,11 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
>   {
>   	if (user_mode(regs)) {
>   		irqentry_enter_from_user_mode(regs);
> +		local_irq_enable();
>   
>   		handle_break(regs);
>   
> +		local_irq_disable();
>   		irqentry_exit_to_user_mode(regs);
>   	} else {
>   		irqentry_state_t state = irqentry_nmi_enter(regs);


Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>

Let's also add:

Cc: stable@...r.kernel.org

I pick this up for next week fixes PR.

Thanks,

Alex