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Date: Fri, 19 Apr 2024 22:33:29 +0200
From: Sam Ravnborg <sam@ravnborg.org>
To: Dawei Li <dawei.li@shingroup.cn>
Cc: davem@davemloft.net, andreas@gaisler.com, sparclinux@vger.kernel.org,
	linux-kernel@vger.kernel.org, yury.norov@gmail.com
Subject: Re: [PATCH 0/5] Remove onstack cpumask var usage
Message-ID: <20240419203329.GA590733@ravnborg.org>
References: <20240418104949.3606645-1-dawei.li@shingroup.cn>
 <20240419051350.GA558245@ravnborg.org>
 <A60F94A9589C8589+ZiI4yj073cgmt5Qq@centos8>
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In-Reply-To: <A60F94A9589C8589+ZiI4yj073cgmt5Qq@centos8>

Hi Dawei,

> About this case, it's kinda tricky for:
> - dr_cpu_data() returns void, so alloc_cpumask_var() is no go.
> 
> - No idea of the calling context of dr_cpu_data(). IIUC,
>   dr_cpu_data()
>   ->dr_cpu_configure()  
>    ->kzalloc(resp_len, GFP_KERNEL)
>   So I guess it's in process context?
>   If consumption above is OK, a simple but _ugly_ solution could be:
> 
> diff --git a/arch/sparc/kernel/ds.c b/arch/sparc/kernel/ds.c
> index ffdc15588ac2..c9e4ebdccf49 100644
> --- a/arch/sparc/kernel/ds.c
> +++ b/arch/sparc/kernel/ds.c
> @@ -634,7 +634,8 @@ static void dr_cpu_data(struct ds_info *dp, struct ds_cap_state *cp, void *buf,
>         struct dr_cpu_tag *tag = (struct dr_cpu_tag *) (data + 1);
>         u32 *cpu_list = (u32 *) (tag + 1);
>         u64 req_num = tag->req_num;
> -       cpumask_t mask;
> +       static DEFINE_MUTEX(mask_lock);
> +       static cpumask_t mask;
>         unsigned int i;
>         int err;
> 
> @@ -651,6 +652,8 @@ static void dr_cpu_data(struct ds_info *dp, struct ds_cap_state *cp, void *buf,
> 
>         purge_dups(cpu_list, tag->num_records);
> 
> +       mutex_lock(&mask_lock);
> +
>         cpumask_clear(&mask);
>         for (i = 0; i < tag->num_records; i++) {
>                 if (cpu_list[i] == CPU_SENTINEL)
> @@ -665,6 +668,8 @@ static void dr_cpu_data(struct ds_info *dp, struct ds_cap_state *cp, void *buf,
>         else
>                 err = dr_cpu_unconfigure(dp, cp, req_num, &mask);
> 
> +       mutex_unlock(&mask_lock);
> +
>         if (err)
>                 dr_cpu_send_error(dp, cp, data);
>  }
> 
> How does it sound to you?
This introduces too much complexity to solve a potential stack issue.
If an improvement is required, then we need a simpler solution.

> 
> > kernel/leon_kernel.c:   cpumask_t mask;
> 
> It's in irqchip::irq_set_affinity(), which is in atomic context(raw spinlock(s) held),
> so dynamic allocation is not a good idea.
> 
> My proposal(*untested*) is somewhat complicated for it introduces a new helper.
> 
> diff --git a/arch/sparc/kernel/leon_kernel.c b/arch/sparc/kernel/leon_kernel.c
> index 4c61da491fee..6eced7acb8bc 100644
> --- a/arch/sparc/kernel/leon_kernel.c
> +++ b/arch/sparc/kernel/leon_kernel.c
> @@ -104,15 +104,25 @@ unsigned long leon_get_irqmask(unsigned int irq)
>  }
> 
>  #ifdef CONFIG_SMP
> +
> +static bool cpumask_include(const struct cpumask *srcp1, const struct cpumask *srcp2)
> +{
> +       unsigned int cpu;
> +
> +       for_each_cpu(cpu, srcp2) {
> +               if (!cpumask_test_cpu(cpu, srcp1))
> +                       return false;
> +       }
> +
> +       return true;
> +}
> +
>  static int irq_choose_cpu(const struct cpumask *affinity)
>  {
> -       cpumask_t mask;
> +       unsigned int cpu = cpumask_first_and(affinity, cpu_online_mask);
> 
> -       cpumask_and(&mask, cpu_online_mask, affinity);
> -       if (cpumask_equal(&mask, cpu_online_mask) || cpumask_empty(&mask))
> -               return boot_cpu_id;
> -       else
> -               return cpumask_first(&mask);
> +       return cpumask_include(affinity, cpu_online_mask) || cpu >= nr_cpu_ids ?
> +              boot_cpu_id : cpu;
>  }

I think something like the following should do the trick.


	if (cpumask_equal(affinity, cpu_online_mask))
		return boot_cpu_id;

	cpuid = cpumask_first_and(affinity, cpu_online_mask);
	if (cpuid < nr_cpu_ids)
		return cpuid;
	else
		return boot_cpu_id;

If the passed affinity equals the online cpu's, then use the boot cpu.
Else, use the first online cpu in the affinity mask.
If none found use the boot cpu.


>  #else
>  #define irq_choose_cpu(affinity) boot_cpu_id
> 
> Is it OK?
> 
> [cc Yury for bitmap API]
> 
> > kernel/leon_smp.c:static void leon_cross_call(void *func, cpumask_t mask, unsigned long arg1,
> > kernel/sun4d_smp.c:static void sun4d_cross_call(void *func, cpumask_t mask, unsigned long arg1,
Looks simple, just pass a pointer and not by value.
> 
> Actually I am awared of existence of (at least some of) them, but so far I
> have not found a _proper_ way of dealing with them(especially for case of
> ds.c).
> 
> Please lemme dig into it.
Looks forward to next iteration.

	Sam