Re: [PATCH 2/4] riscv: consolidate ret_from_kernel_thread into ret_from_fork

From: Guo Ren <guoren@kernel.org>
To: Jisheng Zhang <jszhang@kernel.org>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	 Albert Ou <aou@eecs.berkeley.edu>,
	Nathan Chancellor <nathan@kernel.org>,
	 Nick Desaulniers <ndesaulniers@google.com>,
	linux-riscv@lists.infradead.org,  linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev
Subject: Re: [PATCH 2/4] riscv: consolidate ret_from_kernel_thread into ret_from_fork
Date: Mon, 26 Sep 2022 07:25:30 +0800	[thread overview]
Message-ID: <CAJF2gTTqup62EM9LZQ-9daKk11O0geax9Z3HmTMOxYy2mAwm=w@mail.gmail.com> (raw)
In-Reply-To: <20220925175356.681-3-jszhang@kernel.org>

On Mon, Sep 26, 2022 at 2:03 AM Jisheng Zhang <jszhang@kernel.org> wrote:
>
> The ret_from_kernel_thread() behaves similarly with ret_from_fork(),
> the only difference is whether call the fn(arg) or not, this can be
> acchieved by testing fn is NULL or not, I.E s0 is 0 or not.
>
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
>  arch/riscv/kernel/entry.S   | 11 +++--------
>  arch/riscv/kernel/process.c |  5 ++---
>  2 files changed, 5 insertions(+), 11 deletions(-)
>
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 2207cf44a3bc..a3e1ed2fa2ac 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -323,20 +323,15 @@ END(handle_kernel_stack_overflow)
>
>  ENTRY(ret_from_fork)
>         call schedule_tail
> -       move a0, sp /* pt_regs */
> -       la ra, ret_from_exception
> -       tail syscall_exit_to_user_mode
> -ENDPROC(ret_from_fork)
> -
> -ENTRY(ret_from_kernel_thread)
> -       call schedule_tail
> +       beqz s0, 1f     /* not from kernel thread */
We can't use s0 as condition for ret_from_fork/ret_from_kernel_thread.
The s0=0 is also okay for ret_from_fork.

        /* p->thread holds context to be restored by __switch_to() */
        if (unlikely(args->fn)) {
                /* Kernel thread */
                memset(childregs, 0, sizeof(struct pt_regs));
                childregs->gp = gp_in_global;
                /* Supervisor/Machine, irqs on: */
                childregs->status = SR_PP | SR_PIE;

                p->thread.ra = (unsigned long)ret_from_kernel_thread;
                p->thread.s[0] = (unsigned long)args->fn;
                p->thread.s[1] = (unsigned long)args->fn_arg;
        } else {
                *childregs = *(current_pt_regs());
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                if (usp) /* User fork */
                        childregs->sp = usp;
                if (clone_flags & CLONE_SETTLS)
                        childregs->tp = tls;
                childregs->a0 = 0; /* Return value of fork() */
                p->thread.ra = (unsigned long)ret_from_fork;
        }
        p->thread.sp = (unsigned long)childregs; /* kernel sp */

>         /* Call fn(arg) */
>         move a0, s1
>         jalr s0
> +1:
>         move a0, sp /* pt_regs */
>         la ra, ret_from_exception
>         tail syscall_exit_to_user_mode
> -ENDPROC(ret_from_kernel_thread)
> +ENDPROC(ret_from_fork)
>
>  #ifdef CONFIG_IRQ_STACKS
>  ENTRY(call_on_stack)
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index ceb9ebab6558..67e7cd123ceb 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -34,7 +34,6 @@ EXPORT_SYMBOL(__stack_chk_guard);
>  #endif
>
>  extern asmlinkage void ret_from_fork(void);
> -extern asmlinkage void ret_from_kernel_thread(void);
>
>  void arch_cpu_idle(void)
>  {
> @@ -172,7 +171,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>                 /* Supervisor/Machine, irqs on: */
>                 childregs->status = SR_PP | SR_PIE;
>
> -               p->thread.ra = (unsigned long)ret_from_kernel_thread;
>                 p->thread.s[0] = (unsigned long)args->fn;
>                 p->thread.s[1] = (unsigned long)args->fn_arg;
>         } else {
> @@ -182,8 +180,9 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>                 if (clone_flags & CLONE_SETTLS)
>                         childregs->tp = tls;
>                 childregs->a0 = 0; /* Return value of fork() */
> -               p->thread.ra = (unsigned long)ret_from_fork;
> +               p->thread.s[0] = 0;
>         }
> +       p->thread.ra = (unsigned long)ret_from_fork;
>         p->thread.sp = (unsigned long)childregs; /* kernel sp */
>         return 0;
>  }
> --
> 2.34.1
>


-- 
Best Regards
 Guo Ren