Re: [PATCH v2 07/13] parisc: Replace regular spinlock with spin_trylock on panic path

[Jeroen Roovers <jer@xs4all.nl>]

     Hi Guilherme,

On Tue, 19 Jul 2022 16:53:20 -0300
"Guilherme G. Piccoli" <gpiccoli@igalia.com> 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@HansenPartnership.com>
> Acked-by: Helge Deller <deller@gmx.de> # parisc
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> 
> ---
> 
> V2:
> - Added Helge's ACK - thanks!
> 
>  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 b643092d4b98..7a106008e258
> 100644 --- a/arch/parisc/include/asm/pdc.h
> +++ b/arch/parisc/include/asm/pdc.h
> @@ -83,6 +83,7 @@ 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 6a7e315bcc2e..0e2f70b592f4
> 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 in spin_trylock to prevent deadlock
> + * on panic path.

in => on

>   */
>  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;


Kind regards,
     jer