Re: [PATCH V1] kthread: Unify kernel_thread() and user_mode_thread()

[Huacai Chen]

Hi, all,

I'm very sorry for wasting so much of your time. Although this patch
has dropped, I still agree with Andrews' comment "But the naming isn't
very good anyway.  Should have been usermode_thread/kernel_thread or
user_thread/kernel_thread.". And since Eric describes init and umh as
"user threads run in kernel mode", I want to rename user_mode_thread()
as kmuser_thread(), which is a little better than just user_thread().


Huacai

On Sat, Jun 3, 2023 at 9:53 AM Huacai Chen <chenhuacai@xxxxxxxxxxx> wrote:
>
> Commit 343f4c49f2438d8 ("kthread: Don't allocate kthread_struct for init
> and umh") introduces a new function user_mode_thread() for init and umh.
>
> init and umh are different from typical kernel threads since the don't
> need a "kthread" struct and they will finally become user processes by
> calling kernel_execve(), but on the other hand, they are also different
> from typical user mode threads (they have no "mm" structs at creation
> time, which is traditionally used to distinguish a user thread and a
> kernel thread).
>
> So I think it is reasonable to treat init and umh as "special kernel
> threads". Then let's unify the kernel_thread() and user_mode_thread()
> to kernel_thread() again, and add a new 'user' parameter for init and
> umh.
>
> This also makes code simpler.
>
> Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> ---
> RFC -> V1: Update commit message and change "user" from int to bool.
>
>  include/linux/sched/task.h |  3 +--
>  init/main.c                |  4 ++--
>  kernel/fork.c              | 20 ++------------------
>  kernel/kthread.c           |  2 +-
>  kernel/umh.c               |  6 +++---
>  5 files changed, 9 insertions(+), 26 deletions(-)
>
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index 537cbf9a2ade..02eb953bc809 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -98,8 +98,7 @@ struct task_struct *copy_process(struct pid *pid, int trace, int node,
>  struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node);
>  struct task_struct *fork_idle(int);
>  extern pid_t kernel_thread(int (*fn)(void *), void *arg, const char *name,
> -                           unsigned long flags);
> -extern pid_t user_mode_thread(int (*fn)(void *), void *arg, unsigned long flags);
> +                           unsigned long flags, bool user);
>  extern long kernel_wait4(pid_t, int __user *, int, struct rusage *);
>  int kernel_wait(pid_t pid, int *stat);
>
> diff --git a/init/main.c b/init/main.c
> index af50044deed5..469cebbd35e0 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -697,7 +697,7 @@ noinline void __ref __noreturn rest_init(void)
>          * the init task will end up wanting to create kthreads, which, if
>          * we schedule it before we create kthreadd, will OOPS.
>          */
> -       pid = user_mode_thread(kernel_init, NULL, CLONE_FS);
> +       pid = kernel_thread(kernel_init, NULL, NULL, CLONE_FS, true);
>         /*
>          * Pin init on the boot CPU. Task migration is not properly working
>          * until sched_init_smp() has been run. It will set the allowed
> @@ -710,7 +710,7 @@ noinline void __ref __noreturn rest_init(void)
>         rcu_read_unlock();
>
>         numa_default_policy();
> -       pid = kernel_thread(kthreadd, NULL, NULL, CLONE_FS | CLONE_FILES);
> +       pid = kernel_thread(kthreadd, NULL, NULL, CLONE_FS | CLONE_FILES, false);
>         rcu_read_lock();
>         kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
>         rcu_read_unlock();
> diff --git a/kernel/fork.c b/kernel/fork.c
> index ed4e01daccaa..f91696904252 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2965,7 +2965,7 @@ pid_t kernel_clone(struct kernel_clone_args *args)
>   * Create a kernel thread.
>   */
>  pid_t kernel_thread(int (*fn)(void *), void *arg, const char *name,
> -                   unsigned long flags)
> +                   unsigned long flags, bool user)
>  {
>         struct kernel_clone_args args = {
>                 .flags          = ((lower_32_bits(flags) | CLONE_VM |
> @@ -2974,23 +2974,7 @@ pid_t kernel_thread(int (*fn)(void *), void *arg, const char *name,
>                 .fn             = fn,
>                 .fn_arg         = arg,
>                 .name           = name,
> -               .kthread        = 1,
> -       };
> -
> -       return kernel_clone(&args);
> -}
> -
> -/*
> - * Create a user mode thread.
> - */
> -pid_t user_mode_thread(int (*fn)(void *), void *arg, unsigned long flags)
> -{
> -       struct kernel_clone_args args = {
> -               .flags          = ((lower_32_bits(flags) | CLONE_VM |
> -                                   CLONE_UNTRACED) & ~CSIGNAL),
> -               .exit_signal    = (lower_32_bits(flags) & CSIGNAL),
> -               .fn             = fn,
> -               .fn_arg         = arg,
> +               .kthread        = !user,
>         };
>
>         return kernel_clone(&args);
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 490792b1066e..5f025569eb38 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -400,7 +400,7 @@ static void create_kthread(struct kthread_create_info *create)
>  #endif
>         /* We want our own signal handler (we take no signals by default). */
>         pid = kernel_thread(kthread, create, create->full_name,
> -                           CLONE_FS | CLONE_FILES | SIGCHLD);
> +                           CLONE_FS | CLONE_FILES | SIGCHLD, false);
>         if (pid < 0) {
>                 /* Release the structure when caller killed by a fatal signal. */
>                 struct completion *done = xchg(&create->done, NULL);
> diff --git a/kernel/umh.c b/kernel/umh.c
> index 60aa9e764a38..b0ead7cce761 100644
> --- a/kernel/umh.c
> +++ b/kernel/umh.c
> @@ -130,7 +130,7 @@ static void call_usermodehelper_exec_sync(struct subprocess_info *sub_info)
>
>         /* If SIGCLD is ignored do_wait won't populate the status. */
>         kernel_sigaction(SIGCHLD, SIG_DFL);
> -       pid = user_mode_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD);
> +       pid = kernel_thread(call_usermodehelper_exec_async, sub_info, NULL, SIGCHLD, true);
>         if (pid < 0)
>                 sub_info->retval = pid;
>         else
> @@ -169,8 +169,8 @@ static void call_usermodehelper_exec_work(struct work_struct *work)
>                  * want to pollute current->children, and we need a parent
>                  * that always ignores SIGCHLD to ensure auto-reaping.
>                  */
> -               pid = user_mode_thread(call_usermodehelper_exec_async, sub_info,
> -                                      CLONE_PARENT | SIGCHLD);
> +               pid = kernel_thread(call_usermodehelper_exec_async, sub_info,
> +                                      NULL, CLONE_PARENT | SIGCHLD, true);
>                 if (pid < 0) {
>                         sub_info->retval = pid;
>                         umh_complete(sub_info);
> --
> 2.39.1
>