Re: [PATCH -v4] x86/i8253: Call clockevent_i8253_disable() with interrupts disabled

[Fernando F. Mancera]

On 01/04/2025 11:23, Ingo Molnar wrote:
> I've cleaned up and simplified the changelog, see the patch below.

Thank you. To me it looks good. Should I repost it?

Thanks,
Fernando.

> Thomas, does this look good to you too?
>
> Thanks,
>
> 	Ingo
>
> =========================>
> From: Fernando Fernandez Mancera <ffmancera@xxxxxxxxxx>
> Date: Fri, 28 Mar 2025 00:43:57 +0100
> Subject: [PATCH] x86/i8253: Call clockevent_i8253_disable() with interrupts disabled
>
> There's a lockdep false positive warning related to i8253_lock:
>
>    WARNING: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected
>    ...
>    systemd-sleep/3324 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
>    ffffffffb2c23398 (i8253_lock){+.+.}-{2:2}, at: pcspkr_event+0x3f/0xe0 [pcspkr]
>
>    ...
>    ... which became HARDIRQ-irq-unsafe at:
>    ...
>      lock_acquire+0xd0/0x2f0
>      _raw_spin_lock+0x30/0x40
>      clockevent_i8253_disable+0x1c/0x60
>      pit_timer_init+0x25/0x50
>      hpet_time_init+0x46/0x50
>      x86_late_time_init+0x1b/0x40
>      start_kernel+0x962/0xa00
>      x86_64_start_reservations+0x24/0x30
>      x86_64_start_kernel+0xed/0xf0
>      common_startup_64+0x13e/0x141
>    ...
>
> Lockdep complains due pit_timer_init() using the lock in an IRQ-unsafe
> fashion, but it's a false positive, because there is no deadlock
> possible at that point due to init ordering: at the point where
> pit_timer_init() is called there is no other possible usage of
> i8253_lock because the system is still in the very early boot stage
> with no interrupts.
>
> But in any case, pit_timer_init() should disable interrupts before
> calling clockevent_i8253_disable() out of general principle, and to
> keep lockdep working even in this scenario.
>
> Use scoped_guard() for that, as suggested by Thomas Gleixner.
>
> [ mingo: Cleaned up the changelog. ]
>
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Fernando Fernandez Mancera <ffmancera@xxxxxxxxxx>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20250327234357.3383-1-ffmancera@xxxxxxxxxx
> ---
>   arch/x86/kernel/i8253.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/i8253.c b/arch/x86/kernel/i8253.c
> index 80e262bb627f..cb9852ad6098 100644
> --- a/arch/x86/kernel/i8253.c
> +++ b/arch/x86/kernel/i8253.c
> @@ -46,7 +46,8 @@ bool __init pit_timer_init(void)
>   		 * VMMs otherwise steal CPU time just to pointlessly waggle
>   		 * the (masked) IRQ.
>   		 */
> -		clockevent_i8253_disable();
> +		scoped_guard(irq)
> +			clockevent_i8253_disable();
>   		return false;
>   	}
>   	clockevent_i8253_init(true);