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Date: Wed, 26 Oct 2016 16:14:00 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Andy Lutomirski <luto@...capital.net>
Cc: Roman Pen <roman.penyaev@...fitbricks.com>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [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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