Re: [PATCH] riscv: Always inline on_thread_stack()

[Jisheng Zhang <jszhang@kernel.org>]

On Mon, Oct 12, 2026 at 10:42:11PM +0200, Karl Mehltretter wrote:
> With CONFIG_IRQ_STACKS=y, GCC may outline on_thread_stack().
> CONFIG_KCOV_INSTRUMENT_ALL instruments it, and do_irq() calls it before
> irq_enter_rcu() increments the hardirq count.
> 
> Normal KCOV records this interrupt coverage in the task's buffer, making
> results timing-dependent.
> 
> With CONFIG_KCOV_SELFTEST=y, current->kcov_area remains NULL. In QEMU, a
> timer interrupt called the helper before hardirq accounting. KCOV treated
> it as task coverage and dereferenced NULL. Fault handling recursed to stack
> overflow and stopped boot.
> 
> On a BeagleV Ahead, the unpatched kernel did not finish the selftest within
> 120 seconds. Three patched boots completed it and reached init.
> 
> Mark the helper __always_inline to keep it in noinstr do_irq(), following
> commit 1894a4030582 ("x86: Always inline on_thread_stack() and
> current_top_of_stack()"), which fixed the same out-of-line call from x86
> noinstr code.
> 
> Fixes: 163e76cc6ef4 ("riscv: stack: Support HAVE_IRQ_EXIT_ON_IRQ_STACK")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>


Good catch!
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>

> ---
> Tested with GCC 15.2.0 and QEMU 10.2.1 on v7.3-rc7. Both images
> used byte-identical configs and v7.3-rc7 rebases of these seven pending
> patches (canonical commit IDs shown):
> 
>   3a25097bf845 kcov: use unsigned int for kcov_start() mode parameter
>   e39a9c820c37 kcov: add kcov_pause()/kcov_resume() helpers
>   ae2662563c3e hrtimer: pause KCOV during deferred rearm
>   554af946139e sched: pause KCOV in __schedule()
>   7f608b777204 sched: pause KCOV in try_to_wake_up()
>   2ba73ec2a246 sched: pause KCOV in wake_up_new_task()
>   5ee777070fac context_tracking: exclude from KCOV instrumentation
> 
>                               plain inline  __always_inline
> KCOV selftest started                  yes              yes
> KCOV selftest finished                  no              yes
> Reached /init                           no              yes
> 
> The plain-inline image stopped after printing "kcov: running self test"
> and reached the 45-second limit. The patched image reached /init in two
> 60-second runs. Those runs timed out after the marker because the test
> /init deliberately loops.
> 
> The first common patch is a type prerequisite. The remaining pause and
> exclusion changes suppress separate KCOV callbacks that otherwise obscure
> this test. CONFIG_RISCV_ISA_V_PREEMPTIVE was disabled to avoid a separate
> vector-entry callback. The only A/B source difference was this patch.
> 
> The same images were tested on a BeagleV Ahead (TH1520, four C910 CPUs).
> The plain-inline image did not finish the selftest or reach /init during
> 120 seconds after the start message. It reported RCU stalls, and CPU 1
> did not answer NMI requests. No panic or stack-overflow message was
> printed. All three patched runs completed the selftest in about 0.301
> seconds and reached /init.
> 
>  arch/riscv/include/asm/stacktrace.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/include/asm/stacktrace.h b/arch/riscv/include/asm/stacktrace.h
> index b1495a7e06ce..c7c991452e11 100644
> --- a/arch/riscv/include/asm/stacktrace.h
> +++ b/arch/riscv/include/asm/stacktrace.h
> @@ -16,7 +16,7 @@ extern void notrace walk_stackframe(struct task_struct *task, struct pt_regs *re
>  extern void dump_backtrace(struct pt_regs *regs, struct task_struct *task,
>  			   const char *loglvl);
>  
> -static inline bool on_thread_stack(void)
> +static __always_inline bool on_thread_stack(void)
>  {
>  	return !(((unsigned long)(current->stack) ^ current_stack_pointer) & ~(THREAD_SIZE - 1));
>  }
> 
> base-commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907
> -- 
> 2.53.0
> 
> 
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