Re: [PATCH v2 01/13] ARM: Disable FIQs (but not IRQs) on CPUs shutdown paths

From: "Guilherme G. Piccoli" <gpiccoli@igalia.com>
To: kexec@lists.infradead.org, linux-arm-kernel@lists.infradead.org,
	Marc Zyngier <maz@kernel.org>,
	Russell King <linux@armlinux.org.uk>
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Subject: Re: [PATCH v2 01/13] ARM: Disable FIQs (but not IRQs) on CPUs shutdown paths
Date: Sun, 7 Aug 2022 12:35:56 -0300	[thread overview]
Message-ID: <5b1805ff-eb9f-8575-ceb0-9d2e768f3589@igalia.com> (raw)
In-Reply-To: <20220719195325.402745-2-gpiccoli@igalia.com>

On 19/07/2022 16:53, 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@kernel.org>
> Cc: Michael Kelley <mikelley@microsoft.com>
> Cc: Russell King <linux@armlinux.org.uk>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> 
> ---
> 
> 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(-)
> [...]

Hi Mark / Russell, do you think this one is good enough or is there room
for improvement?

Appreciate the reviews!
Cheers,

Guilherme

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