linux-kernel - Re: [PATCH V3 01/11] ARM: Disable FIQs (but not IRQs) on CPUs shutdown paths

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Date:   Sun, 18 Sep 2022 10:58:34 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     Russell King <linux@...linux.org.uk>,
        Marc Zyngier <maz@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, will@...nel.org,
        Mark Rutland <mark.rutland@....com>, arnd@...db.de,
        Catalin Marinas <catalin.marinas@....com>
Cc:     kexec@...ts.infradead.org, pmladek@...e.com, bhe@...hat.com,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, netdev@...r.kernel.org,
        x86@...nel.org, kernel-dev@...lia.com, kernel@...ccoli.net,
        halves@...onical.com, fabiomirmar@...il.com,
        alejandro.j.jimenez@...cle.com, andriy.shevchenko@...ux.intel.com,
        bp@...en8.de, corbet@....net, d.hatayama@...fujitsu.com,
        dave.hansen@...ux.intel.com, dyoung@...hat.com,
        feng.tang@...el.com, gregkh@...uxfoundation.org,
        mikelley@...rosoft.com, hidehiro.kawai.ez@...achi.com,
        jgross@...e.com, john.ogness@...utronix.de, keescook@...omium.org,
        luto@...nel.org, mhiramat@...nel.org, mingo@...hat.com,
        paulmck@...nel.org, peterz@...radead.org, rostedt@...dmis.org,
        senozhatsky@...omium.org, stern@...land.harvard.edu,
        tglx@...utronix.de, vgoyal@...hat.com, vkuznets@...hat.com,
        xuqiang36@...wei.com
Subject: Re: [PATCH V3 01/11] ARM: Disable FIQs (but not IRQs) on CPUs
 shutdown paths

On 19/08/2022 19:17, Guilherme G. Piccoli wrote:
> Currently the regular CPU shutdown path for ARM disables IRQs/FIQs
> in the secondary CPUs - smp_send_stop() calls ipi_cpu_stop(), which
> is responsible for that. IRQs are architecturally masked when we
> take an interrupt, but FIQs are high priority than IRQs, hence they
> aren't masked. With that said, it makes sense to disable FIQs here,
> but there's no need for (re-)disabling IRQs.
> 
> More than that: there is an alternative path for disabling CPUs,
> in the form of function crash_smp_send_stop(), which is used for
> kexec/panic path. This function relies on a SMP call that also
> triggers a busy-wait loop [at machine_crash_nonpanic_core()], but
> without disabling FIQs. This might lead to odd scenarios, like
> early interrupts in the boot of kexec'd kernel or even interrupts
> in secondary "disabled" CPUs while the main one still works in the
> panic path and assumes all secondary CPUs are (really!) off.
> 
> So, let's disable FIQs in both paths and *not* disable IRQs a second
> time, since they are already masked in both paths by the architecture.
> This way, we keep both CPU quiesce paths consistent and safe.
> 
> Cc: Marc Zyngier <maz@...nel.org>
> Cc: Michael Kelley <mikelley@...rosoft.com>
> Cc: Russell King <linux@...linux.org.uk>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>
> 
> ---
> 
> V3:
> - No changes.
> 
> V2:
> - Small wording improvement (thanks Michael Kelley);
> - Only disable FIQs, since IRQs are masked by architecture
> definition when we take an interrupt. Thanks a lot Russell
> and Marc for the discussion [0].
> 
> Should we add a Fixes tag here? If so, maybe the proper target is:
> b23065313297 ("ARM: 6522/1: kexec: Add call to non-crashing cores through IPI")
> 
> [0] https://lore.kernel.org/lkml/Ymxcaqy6DwhoQrZT@shell.armlinux.org.uk/
> 
> 
>  arch/arm/kernel/machine_kexec.c | 2 ++
>  arch/arm/kernel/smp.c           | 5 ++---
>  2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm/kernel/machine_kexec.c b/arch/arm/kernel/machine_kexec.c
> index f567032a09c0..0b482bcb97f7 100644
> --- a/arch/arm/kernel/machine_kexec.c
> +++ b/arch/arm/kernel/machine_kexec.c
> @@ -77,6 +77,8 @@ void machine_crash_nonpanic_core(void *unused)
>  {
>  	struct pt_regs regs;
>  
> +	local_fiq_disable();
> +
>  	crash_setup_regs(&regs, get_irq_regs());
>  	printk(KERN_DEBUG "CPU %u will stop doing anything useful since another CPU has crashed\n",
>  	       smp_processor_id());
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 978db2d96b44..36e6efad89f3 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -600,6 +600,8 @@ static DEFINE_RAW_SPINLOCK(stop_lock);
>   */
>  static void ipi_cpu_stop(unsigned int cpu)
>  {
> +	local_fiq_disable();
> +
>  	if (system_state <= SYSTEM_RUNNING) {
>  		raw_spin_lock(&stop_lock);
>  		pr_crit("CPU%u: stopping\n", cpu);
> @@ -609,9 +611,6 @@ static void ipi_cpu_stop(unsigned int cpu)
>  
>  	set_cpu_online(cpu, false);
>  
> -	local_fiq_disable();
> -	local_irq_disable();
> -
>  	while (1) {
>  		cpu_relax();
>  		wfe();

[+CC all ARM folks I could find]

Hi folks, sorry for the ping. Any reviews are greatly appreciated in
this one - this is the V3 of the series but I never got any specific
review for this patch.

Based on a previous discussion with Russell and Marc [0], seems this one
makes sense and fixes an inconsistency related to kdump/panic.

Really appreciate reviews or at least some indication on which path I
should take to get this potentially merged.

Cheers,


Guilherme


[0] https://lore.kernel.org/lkml/Ymxcaqy6DwhoQrZT@shell.armlinux.org.uk/