linux-kernel - Re: [PATCH v6 2/5] LoongArch: Use stop_machine

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Date:   Fri, 2 Dec 2022 15:09:43 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     WANG Xuerui <kernel@...0n.name>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/5] LoongArch: Use stop_machine_cpuslocked() to modify instruction

Hi, Tiezhu,

On Wed, Nov 30, 2022 at 4:19 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> Rename the current larch_insn_patch_text() to patch_text_nosync(),
> then redefine larch_insn_patch_text() with stop_machine_cpuslocked()
> to modify instruction to avoid CPU race, this is preparation for later
> patch.
I don't think stop_machine is a good idea for this, because it is too
expensive, maybe Masami can give us a better suggestion.

Huacai
>
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  arch/loongarch/kernel/inst.c | 33 ++++++++++++++++++++++++++++++++-
>  1 file changed, 32 insertions(+), 1 deletion(-)
>
> diff --git a/arch/loongarch/kernel/inst.c b/arch/loongarch/kernel/inst.c
> index aaaf9de..d842405 100644
> --- a/arch/loongarch/kernel/inst.c
> +++ b/arch/loongarch/kernel/inst.c
> @@ -3,6 +3,7 @@
>   * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
>   */
>  #include <linux/sizes.h>
> +#include <linux/stop_machine.h>
>  #include <linux/uaccess.h>
>
>  #include <asm/cacheflush.h>
> @@ -10,6 +11,12 @@
>
>  static DEFINE_RAW_SPINLOCK(patch_lock);
>
> +struct patch_insn {
> +       void *addr;
> +       u32 insn;
> +       atomic_t cpu_count;
> +};
> +
>  int larch_insn_read(void *addr, u32 *insnp)
>  {
>         int ret;
> @@ -34,7 +41,7 @@ int larch_insn_write(void *addr, u32 insn)
>         return ret;
>  }
>
> -int larch_insn_patch_text(void *addr, u32 insn)
> +static int patch_text_nosync(void *addr, u32 insn)
>  {
>         int ret;
>         u32 *tp = addr;
> @@ -50,6 +57,30 @@ int larch_insn_patch_text(void *addr, u32 insn)
>         return ret;
>  }
>
> +static int patch_text_cb(void *data)
> +{
> +       struct patch_insn *param = data;
> +       int ret = 0;
> +
> +       if (atomic_inc_return(&param->cpu_count) == num_online_cpus()) {
> +               ret = patch_text_nosync(param->addr, param->insn);
> +               atomic_inc(&param->cpu_count);
> +       } else {
> +               while (atomic_read(&param->cpu_count) <= num_online_cpus())
> +                       cpu_relax();
> +               smp_mb();
> +       }
> +
> +       return ret;
> +}
> +
> +int larch_insn_patch_text(void *addr, u32 insn)
> +{
> +       struct patch_insn param = { addr, insn, ATOMIC_INIT(0) };
> +
> +       return stop_machine_cpuslocked(patch_text_cb, &param, cpu_online_mask);
> +}
> +
>  u32 larch_insn_gen_nop(void)
>  {
>         return INSN_NOP;
> --
> 2.1.0
>
>