Re: [PATCH v4 03/14] irqchip: mips-gic: Introduce for_each_online_cpu_gic()
From: Thomas Gleixner
Date: Fri Jun 21 2024 - 14:58:52 EST
On Sat, May 11 2024 at 12:43, Aleksandar Rikalo wrote:
> A few pieces of code in the MIPS GIC driver operate on the GIC local
> register block for each online CPU, accessing each via the GIC's
> other/redirect register block. This patch abstracts the process of
> iterating over online CPUs & configuring the other/redirect region to
> access their registers through a new for_each_online_cpu_gic() macro.
>
> This simplifies users of the new macro slightly, and more importantly
> prepares us for handling multi-cluster systems where the register
> configuration will be done via the CM's GCR_CL_REDIRECT register. By
> abstracting all other/redirect block configuration through this macro,
> and the __gic_with_next_online_cpu() function which backs it, users will
> trivially gain support for multi-cluster when it is implemented in
> __gic_with_next_online_cpu().
Can you please rework the change log according to
https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#changelog
> Signed-off-by: Paul Burton <paulburton@xxxxxxxxxx>
> Signed-off-by: Chao-ying Fu <cfu@xxxxxxxxxxxx>
> Signed-off-by: Dragan Mladjenovic <dragan.mladjenovic@xxxxxxxxxx>
> Signed-off-by: Aleksandar Rikalo <aleksandar.rikalo@xxxxxxxxxx>
> ---
> drivers/irqchip/irq-mips-gic.c | 61 +++++++++++++++++++++++++++++-----
> 1 file changed, 52 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/irqchip/irq-mips-gic.c b/drivers/irqchip/irq-mips-gic.c
> index 76253e864f23..9e7182150b5c 100644
> --- a/drivers/irqchip/irq-mips-gic.c
> +++ b/drivers/irqchip/irq-mips-gic.c
> @@ -66,6 +66,52 @@ static struct gic_all_vpes_chip_data {
> bool mask;
> } gic_all_vpes_chip_data[GIC_NUM_LOCAL_INTRS];
>
> +static int __gic_with_next_online_cpu(int prev)
> +{
> + unsigned int cpu;
> +
> + /* Discover the next online CPU */
> + cpu = cpumask_next(prev, cpu_online_mask);
> +
> + /* If there isn't one, we're done */
> + if (cpu >= nr_cpu_ids)
> + return cpu;
> +
> + /*
> + * Lock access to the next CPU's GIC local register block.
> + *
> + * In the single cluster case we simply set GIC_VL_OTHER. The caller
> + * holds gic_lock so nothing can clobber the value we write.
> + */
> + write_gic_vl_other(mips_cm_vp_id(cpu));
What unlocks the access? I assume it's magic, but then magic wants to be
documented.
> +
> + return cpu;
> +}
> +
> +static inline void __lockdep_assert_held(raw_spinlock_t *gic_lock)
> +{
> + lockdep_assert_held(gic_lock);
> +}
What is exactly the point of this indirection?
> +/**
> + * for_each_online_cpu_gic() - Iterate over online CPUs, access local registers
> + * @cpu: An integer variable to hold the current CPU number
> + * @gic_lock: A pointer to raw spin lock used as a guard
> + *
> + * Iterate over online CPUs & configure the other/redirect register region to
> + * access each CPUs GIC local register block, which can be accessed from the
> + * loop body using read_gic_vo_*() or write_gic_vo_*() accessor functions or
> + * their derivatives.
> + *
> + * The caller must hold gic_lock throughout the loop, such that GIC_VL_OTHER
> + * cannot be clobbered.
> + */
> +#define for_each_online_cpu_gic(cpu, gic_lock) \
> + for (__lockdep_assert_held(gic_lock), \
> + (cpu) = __gic_with_next_online_cpu(-1); \
> + (cpu) = __gic_with_next_online_cpu(cpu), \
> + (cpu) < nr_cpu_ids;)
That's broken. It resolves to:
for (cpu = foo(-1); cpu = foo(cpu), cpu < nr_cpu_ids; )
So on entering the loop:
cpu = foo(-1); -> cpu == 0
Now it has to evaluate the loop condition which does:
cpu = foo(cpu) -> cpu == 1
...
So CPU 0 is skipped unconditionally. No?
Aside of that. Instead of this __lockdep_assert_held() obfuscation you
can simply do:
#define for_each_online_cpu_gic(cpu, gic_lock) \
guard(raw_spinlock_irqsave)(gic_lock); \
for ((cpu) = __gic_with_next_online_cpu(-1); \
(cpu) < nr_cpu_ids; \
(cpu) = __gic_with_next_online_cpu(cpu);)
which simplifies the callsites even further.
Thanks,
tglx