Re: [PATCH v2] LoongArch: Fix calling smp_processor_id() in preemptible code

[Hengqi Chen]

On Fri, Feb 27, 2026 at 4:04 PM Xi Ruoyao <xry111@xxxxxxxxxxx> wrote:
>
> Fix the warning:
>
>     BUG: using smp_processor_id() in preemptible [00000000] code: sys
> temd/1
>     caller is larch_insn_text_copy+0x40/0xf0
>
> Simply changing it to raw_smp_processor_id() is not enough: if preempt
> and CPU hotplug happens after raw_smp_processor_id() but before
> stop_machine(), the CPU where raw_smp_processor_id() has run may be
> offline when stop_machine() and no CPU will run copy_to_kernel_nofault()
> in text_copy_cb().  Thus guard the larch_insn_text_copy() calls with
> cpus_read_lock() and change stop_machine() to stop_machine_cpuslocked()
> to prevent this.
>

I think you should include the P.S. part from v1 in the commit message.

> Fixes: 9fbd18cf4c69 ("LoongArch: BPF: Add dynamic code modification support")
> Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx>
> ---
>
> Change since [v1]:
> - Add lockdep_assert_cpus_held() with a comment to prevent people from
>   calling larch_insn_text_copy() w/o acquiring the cpus lock in the
>   future.
>
> [v1]:https://lore.kernel.org/20260225141404.178823-2-xry111@xxxxxxxxxxx
>
>  arch/loongarch/kernel/inst.c | 10 ++++++++--
>  arch/loongarch/net/bpf_jit.c |  6 ++++++
>  2 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/arch/loongarch/kernel/inst.c b/arch/loongarch/kernel/inst.c
> index bf037f0c6b26..7545ae3c796e 100644
> --- a/arch/loongarch/kernel/inst.c
> +++ b/arch/loongarch/kernel/inst.c
> @@ -263,14 +263,20 @@ int larch_insn_text_copy(void *dst, void *src, size_t len)
>                 .dst = dst,
>                 .src = src,
>                 .len = len,
> -               .cpu = smp_processor_id(),
> +               .cpu = raw_smp_processor_id(),
>         };
>
> +       /*
> +        * Ensure copy.cpu won't be hot removed before stop_machine.  If
> +        * it's removed nobody will really update the text.
> +        */
> +       lockdep_assert_cpus_held();
> +
>         start = round_down((size_t)dst, PAGE_SIZE);
>         end   = round_up((size_t)dst + len, PAGE_SIZE);
>
>         set_memory_rw(start, (end - start) / PAGE_SIZE);
> -       ret = stop_machine(text_copy_cb, &copy, cpu_online_mask);
> +       ret = stop_machine_cpuslocked(text_copy_cb, &copy, cpu_online_mask);
>         set_memory_rox(start, (end - start) / PAGE_SIZE);
>
>         return ret;
> diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
> index 3bd89f55960d..e8e0ad34928c 100644
> --- a/arch/loongarch/net/bpf_jit.c
> +++ b/arch/loongarch/net/bpf_jit.c
> @@ -1379,9 +1379,11 @@ void *bpf_arch_text_copy(void *dst, void *src, size_t len)
>  {
>         int ret;
>
> +       cpus_read_lock();
>         mutex_lock(&text_mutex);
>         ret = larch_insn_text_copy(dst, src, len);
>         mutex_unlock(&text_mutex);
> +       cpus_read_unlock();
>
>         return ret ? ERR_PTR(-EINVAL) : dst;
>  }
> @@ -1429,10 +1431,12 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type old_t,
>         if (ret)
>                 return ret;
>
> +       cpus_read_lock();
>         mutex_lock(&text_mutex);
>         if (memcmp(ip, new_insns, LOONGARCH_LONG_JUMP_NBYTES))
>                 ret = larch_insn_text_copy(ip, new_insns, LOONGARCH_LONG_JUMP_NBYTES);
>         mutex_unlock(&text_mutex);
> +       cpus_read_unlock();
>
>         return ret;
>  }
> @@ -1450,10 +1454,12 @@ int bpf_arch_text_invalidate(void *dst, size_t len)
>         for (i = 0; i < (len / sizeof(u32)); i++)
>                 inst[i] = INSN_BREAK;
>
> +       cpus_read_lock();
>         mutex_lock(&text_mutex);
>         if (larch_insn_text_copy(dst, inst, len))
>                 ret = -EINVAL;
>         mutex_unlock(&text_mutex);
> +       cpus_read_unlock();
>
>         kvfree(inst);
>
> --
> 2.53.0
>