Re: [PATCH 1/3] powerpc/time: remove preempt_disable/enable from arch_irq_work_raise()

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 5/6/26 2:36 PM, Sayali Patil wrote:
> A kernel panic is observed when handling machine check exceptions from
> real mode.
> 
>    BUG: Unable to handle kernel data access on read at 0xc00000006be21300
>    Oops: Kernel access of bad area, sig: 11 [#1]
>    NIP [c000000000029e40] arch_irq_work_raise+0x10/0x70
>    LR [c00000000003ffc8] machine_check_queue_event+0xa8/0x150
>    Call Trace:
>    [c0000000179d3c70] [c00000000003ff64] machine_check_queue_event+0x44/0x150
>    [c0000000179d3d30] [c0000000000084e0] machine_check_early_common+0x1f0/0x2c0
> 
> The crash occurs because arch_irq_work_raise() calls preempt_disable()
> from machine check exception (MCE) handlers running in real mode. In
> this context, accessing the preempt_count can fault, leading to the panic.
> 
> The preempt_disable()/preempt_enable() pair in arch_irq_work_raise()
> was originally added by commit 0fe1ac48bef0 ("powerpc/perf_event: Fix
> oops due to perf_event_do_pending call") to avoid races while raising
> irq work from exception context.
> 
> Later, commit 471ba0e686cb ("irq_work: Do not raise an IPI when
> queueing work on the local CPU") added preemption protection in
> irq_work_queue() path, while commit 20b876918c06 ("irq_work: Use per
> cpu atomics instead of regular atomics") added equivalent
> protection in irq_work_queue_on() before reaching arch_irq_work_raise():
> 
>    irq_work_queue() / irq_work_queue_on()
>      -> preempt_disable()
>        -> __irq_work_queue_local()
>          -> irq_work_raise()
>            -> arch_irq_work_raise()
> 
> As a result, callers other than mce_irq_work_raise() already execute
> with preemption disabled, making the additional
> preempt_disable()/preempt_enable() pair in arch_irq_work_raise()
> redundant.
> 
> Remove it to avoid accessing preempt_count from real mode context.

I assume interrupt is disabled here. So it should be functionally safe
to remove it.

> 
> Fixes: cc15ff327569 ("powerpc/mce: Avoid using irq_work_queue() in realmode")
> Suggested-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
> Signed-off-by: Sayali Patil <sayalip@linux.ibm.com>
> ---
>   arch/powerpc/kernel/time.c | 2 --
>   1 file changed, 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index 4bbeb8644d3d..a99eb43f6ce9 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -471,10 +471,8 @@ void arch_irq_work_raise(void)

Could you please add a comment for the function that it expects to
be called with preemption_disabled?

>   	 * which could get tangled up if we're messing with the same state
>   	 * here.
>   	 */
> -	preempt_disable();
>   	set_irq_work_pending_flag();
>   	set_dec(1);
> -	preempt_enable();
>   }
>   
>   static void set_dec_or_work(u64 val)

Acked-by: Shrikanth Hegde <sshegde@linux.ibm.com>