Date:   Tue, 28 Feb 2023 19:45:12 +0100
From:   Helge Deller <deller@....de>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        linux-parisc@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-dev@...lia.com,
        kernel@...ccoli.net,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Jeroen Roovers <jer@...all.nl>
Subject: Re: [PATCH v4] parisc: Replace regular spinlock with spin_trylock on
 panic path

On 2/20/23 22:11, Guilherme G. Piccoli wrote:
> The panic notifiers' callbacks execute in an atomic context, with
> interrupts/preemption disabled, and all CPUs not running the panic
> function are off, so it's very dangerous to wait on a regular
> spinlock, there's a risk of deadlock.
>
> Refactor the panic notifier of parisc/power driver to make use
> of spin_trylock - for that, we've added a second version of the
> soft-power function. Also, some comments were reorganized and
> trailing white spaces, useless header inclusion and blank lines
> were removed.
>
> Cc: "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>
> Cc: Jeroen Roovers <jer@...all.nl>
> Acked-by: Helge Deller <deller@....de> # parisc
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>
>
> ---
> V4:
> - rebase against v6.2 (build-tested).
>
> V3:
> - s/in/on as per Jeroen's suggestion - thanks!
>
> V2:
> - Added Helge's ACK - thanks!
>
>
> Hey Helge, this is the V4 - just rebased and build-tested on v6.2.
> I remember we discussed the possibility of this getting merged through
> the series I've submitted [0], but happens the series is "gone", most of
> the patches were picked by maintainers, others I'm dropping. This is really
> a standalone fix, similar ones were merged in other notifiers.
>
> I'd appreciate if you/James could pick it for the next kernel, there's no
> rush at all, but there's also no series we could throw this patch to get
> merged with heh
>
> Thanks in advance,
> Guilherme

Thank you Guilherme!

I've now picked it up in the parisc-for-next tree.

Helge


>
> [0] https://lore.kernel.org/lkml/43de5653-7587-3e8e-274c-b2729649d0fd@gmx.de/
>
>
>   arch/parisc/include/asm/pdc.h |  1 +
>   arch/parisc/kernel/firmware.c | 27 +++++++++++++++++++++++----
>   drivers/parisc/power.c        | 17 ++++++++++-------
>   3 files changed, 34 insertions(+), 11 deletions(-)
>
> diff --git a/arch/parisc/include/asm/pdc.h b/arch/parisc/include/asm/pdc.h
> index 40793bef8429..2b4fad8328e8 100644
> --- a/arch/parisc/include/asm/pdc.h
> +++ b/arch/parisc/include/asm/pdc.h
> @@ -80,6 +80,7 @@ int pdc_do_firm_test_reset(unsigned long ftc_bitmap);
>   int pdc_do_reset(void);
>   int pdc_soft_power_info(unsigned long *power_reg);
>   int pdc_soft_power_button(int sw_control);
> +int pdc_soft_power_button_panic(int sw_control);
>   void pdc_io_reset(void);
>   void pdc_io_reset_devices(void);
>   int pdc_iodc_getc(void);
> diff --git a/arch/parisc/kernel/firmware.c b/arch/parisc/kernel/firmware.c
> index 6817892a2c58..cc124d9f1f7f 100644
> --- a/arch/parisc/kernel/firmware.c
> +++ b/arch/parisc/kernel/firmware.c
> @@ -1232,15 +1232,18 @@ int __init pdc_soft_power_info(unsigned long *power_reg)
>   }
>
>   /*
> - * pdc_soft_power_button - Control the soft power button behaviour
> - * @sw_control: 0 for hardware control, 1 for software control
> + * pdc_soft_power_button{_panic} - Control the soft power button behaviour
> + * @sw_control: 0 for hardware control, 1 for software control
>    *
>    *
>    * This PDC function places the soft power button under software or
>    * hardware control.
> - * Under software control the OS may control to when to allow to shut
> - * down the system. Under hardware control pressing the power button
> + * Under software control the OS may control to when to allow to shut
> + * down the system. Under hardware control pressing the power button
>    * powers off the system immediately.
> + *
> + * The _panic version relies on spin_trylock to prevent deadlock
> + * on panic path.
>    */
>   int pdc_soft_power_button(int sw_control)
>   {
> @@ -1254,6 +1257,22 @@ int pdc_soft_power_button(int sw_control)
>   	return retval;
>   }
>
> +int pdc_soft_power_button_panic(int sw_control)
> +{
> +	int retval;
> +	unsigned long flags;
> +
> +	if (!spin_trylock_irqsave(&pdc_lock, flags)) {
> +		pr_emerg("Couldn't enable soft power button\n");
> +		return -EBUSY; /* ignored by the panic notifier */
> +	}
> +
> +	retval = mem_pdc_call(PDC_SOFT_POWER, PDC_SOFT_POWER_ENABLE, __pa(pdc_result), sw_control);
> +	spin_unlock_irqrestore(&pdc_lock, flags);
> +
> +	return retval;
> +}
> +
>   /*
>    * pdc_io_reset - Hack to avoid overlapping range registers of Bridges devices.
>    * Primarily a problem on T600 (which parisc-linux doesn't support) but
> diff --git a/drivers/parisc/power.c b/drivers/parisc/power.c
> index 456776bd8ee6..8512884de2cf 100644
> --- a/drivers/parisc/power.c
> +++ b/drivers/parisc/power.c
> @@ -37,7 +37,6 @@
>   #include <linux/module.h>
>   #include <linux/init.h>
>   #include <linux/kernel.h>
> -#include <linux/notifier.h>
>   #include <linux/panic_notifier.h>
>   #include <linux/reboot.h>
>   #include <linux/sched/signal.h>
> @@ -175,16 +174,21 @@ static void powerfail_interrupt(int code, void *x)
>
>
>
> -/* parisc_panic_event() is called by the panic handler.
> - * As soon as a panic occurs, our tasklets above will not be
> - * executed any longer. This function then re-enables the
> - * soft-power switch and allows the user to switch off the system
> +/*
> + * parisc_panic_event() is called by the panic handler.
> + *
> + * As soon as a panic occurs, our tasklets above will not
> + * be executed any longer. This function then re-enables
> + * the soft-power switch and allows the user to switch off
> + * the system. We rely in pdc_soft_power_button_panic()
> + * since this version spin_trylocks (instead of regular
> + * spinlock), preventing deadlocks on panic path.
>    */
>   static int parisc_panic_event(struct notifier_block *this,
>   		unsigned long event, void *ptr)
>   {
>   	/* re-enable the soft-power switch */
> -	pdc_soft_power_button(0);
> +	pdc_soft_power_button_panic(0);
>   	return NOTIFY_DONE;
>   }
>
> @@ -193,7 +197,6 @@ static struct notifier_block parisc_panic_block = {
>   	.priority	= INT_MAX,
>   };
>
> -
>   static int __init power_init(void)
>   {
>   	unsigned long ret;

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