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    SubjectRe: [PATCH v3 1/1] kthread: allocate kthread structure using kmalloc
    On 10/25, Andy Lutomirski wrote:
    >
    > Would it perhaps make sense to do something like Roman's patch for 4.9
    > and then consider further changes down the road?

    OK, lets make it kmalloc'ed. Please see my old patch below, slightly changed.

    I'll send it after I do some testing and write the changelog. No separate
    refcounting, no complications.

    Some notes right now. Of course, with this patch we are ready to remove
    put_task_stack() from kthread.c right now. The next change should kill
    to_live_kthread() altogether. And stop using ->vfork_done.

    And. With this patch we do not need another "workqueue: ignore dead tasks
    in a workqueue sleep hook" fix from Roman.

    > Roman's patch
    > appears to fix a real bug,

    Well, it fixes the additional problems if we already have a bug, but I
    agree this is a problem anyway.

    > and I think that, while not really ideal,
    > the code is an incredible mess right now and Roman's patch (assuming
    > it's correct) makes it considerably nicer.

    This is where I disagree with you and Roman. Yes, it needs cleanups and
    only because of kthread on stack. But _IMO_ Roman's patch makes it much,
    much worse and adds a lot of unnecessary complications.

    Could you please look at the patch below?

    Oleg.
    ---

    diff --git a/include/linux/kthread.h b/include/linux/kthread.h
    index a6e82a6..c1c3e63 100644
    --- a/include/linux/kthread.h
    +++ b/include/linux/kthread.h
    @@ -48,6 +48,7 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
    __k; \
    })

    +void free_kthread_struct(struct task_struct *k);
    void kthread_bind(struct task_struct *k, unsigned int cpu);
    void kthread_bind_mask(struct task_struct *k, const struct cpumask *mask);
    int kthread_stop(struct task_struct *k);
    diff --git a/kernel/fork.c b/kernel/fork.c
    index 623259f..663c6a7 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -351,6 +351,8 @@ void free_task(struct task_struct *tsk)
    ftrace_graph_exit_task(tsk);
    put_seccomp_filter(tsk);
    arch_release_task_struct(tsk);
    + if (tsk->flags & PF_KTHREAD)
    + free_kthread_struct(tsk);
    free_task_struct(tsk);
    }
    EXPORT_SYMBOL(free_task);
    diff --git a/kernel/kthread.c b/kernel/kthread.c
    index be2cc1f..c6adbde 100644
    --- a/kernel/kthread.c
    +++ b/kernel/kthread.c
    @@ -53,14 +53,34 @@ enum KTHREAD_BITS {
    KTHREAD_IS_PARKED,
    };

    -#define __to_kthread(vfork) \
    - container_of(vfork, struct kthread, exited)
    +static inline void set_kthread_struct(void *kthread)
    +{
    + /*
    + * We abuse ->set_child_tid to avoid the new member and because it
    + * can't be wrongly copied by copy_process(). We also rely on fact
    + * that the caller can't exec, so PF_KTHREAD can't be cleared.
    + */
    + current->set_child_tid = (__force void __user *)kthread;
    +}

    static inline struct kthread *to_kthread(struct task_struct *k)
    {
    - return __to_kthread(k->vfork_done);
    + WARN_ON(!(k->flags & PF_KTHREAD));
    + return (__force void *)k->set_child_tid;
    }

    +void free_kthread_struct(struct task_struct *k)
    +{
    + kfree(to_kthread(k)); /* can be NULL if kmalloc() failed */
    +}
    +
    +#define __to_kthread(vfork) \
    + container_of(vfork, struct kthread, exited)
    +
    +/*
    + * TODO: kill it and use to_kthread(). But we still need the users
    + * like kthread_stop() which has to sync with the exiting kthread.
    + */
    static struct kthread *to_live_kthread(struct task_struct *k)
    {
    struct completion *vfork = ACCESS_ONCE(k->vfork_done);
    @@ -181,14 +201,11 @@ static int kthread(void *_create)
    int (*threadfn)(void *data) = create->threadfn;
    void *data = create->data;
    struct completion *done;
    - struct kthread self;
    + struct kthread *self;
    int ret;

    - self.flags = 0;
    - self.data = data;
    - init_completion(&self.exited);
    - init_completion(&self.parked);
    - current->vfork_done = &self.exited;
    + self = kmalloc(sizeof(*self), GFP_KERNEL);
    + set_kthread_struct(self);

    /* If user was SIGKILLed, I release the structure. */
    done = xchg(&create->done, NULL);
    @@ -196,6 +213,19 @@ static int kthread(void *_create)
    kfree(create);
    do_exit(-EINTR);
    }
    +
    + if (!self) {
    + create->result = ERR_PTR(-ENOMEM);
    + complete(done);
    + do_exit(-ENOMEM);
    + }
    +
    + self->flags = 0;
    + self->data = data;
    + init_completion(&self->exited);
    + init_completion(&self->parked);
    + current->vfork_done = &self->exited;
    +
    /* OK, tell user we're spawned, wait for stop or wakeup */
    __set_current_state(TASK_UNINTERRUPTIBLE);
    create->result = current;
    @@ -203,12 +233,10 @@ static int kthread(void *_create)
    schedule();

    ret = -EINTR;
    -
    - if (!test_bit(KTHREAD_SHOULD_STOP, &self.flags)) {
    - __kthread_parkme(&self);
    + if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
    + __kthread_parkme(self);
    ret = threadfn(data);
    }
    - /* we can't just return, we must preserve "self" on stack */
    do_exit(ret);
    }

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