Re: [PATCH 12/30] parisc: Replace regular spinlock with spin_trylock on panic path

[Helge Deller <deller@xxxxxx>]

On 4/28/22 00:49, 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.
>
> This patch refactors 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: Helge Deller <deller@xxxxxx>
> Cc: "James E.J. Bottomley" <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx>

You may add:
Acked-by: Helge Deller <deller@xxxxxx> # parisc

Helge


> ---
>  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.
>   */
>  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;