Re: [PATCH 3/3] make kthread_stop() scalable

[Oleg Nesterov]

On 04/13, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@xxxxxxxxxx> writes:
> 
> > It's a shame kthread_stop() (may take a while!) runs with a global semaphore
> > held. With this patch kthread() allocates all neccesary data (struct kthread)
> > on its own stack, globals kthread_stop_xxx are deleted.
> 
> Oleg so fare you patches  have been inspiring.  However..
> 
> > HACKS:
> >
> > 	- re-use task_struct->set_child_tid to point to "struct kthread"
> 
> 	 task_struct->vfork_done is a better cannidate.
> 
> > 	- use do_exit() directly to preserve "struct kthread" on stack
> 
> Calling do_exit directly like that is not a hack, as it appears the preferred
> way to exit is to call do_exit, or complete_and_exit.
> 
> While this does improve the scalability and remove a global variable.  It
> also introduces a complex special case in the form of struct kthread.

I can't say I agree. I thought it is good to have a struct which represents
a kernel thread. Actually, I thought we can have __kthread_create() which
returns "struct kthread". May be I am wrong, because yes, ->set_child_tid can
point right to completion, and we can use some TIF flag instead of ->should_stop.
This needs to update a lot of include/asm/ files.

> It also doesn't solve the biggest problem with the current kthread interface
> in that calling kthread_stop does not cause the code to break out of
> interruptible sleeps.

Hm? kthread_stop() does wake_up_process(), it wakes up TASK_INTERRUPTIBLE tasks.

> > @@ -91,7 +105,7 @@ static void create_kthread(struct kthrea
> >
> >  	/* We want our own signal handler (we take no signals by default). */
> >  	pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
> > -	create->result = pid;
> > +	create->result = ERR_PTR(pid);
> 
> Ouch.    You have a nasty race here.
> 
> If kthread runs before kernel_thread returns then setting
> "create->result = ERR_PTR(pid);" could easily stomp 
> "create->result = &self".

Yes, thanks... Can't understand how I was soooo stupid!!! thanks...

Damn. We don't need 2 completions! just one.

	create_kthread:

		pid = kernel_thread(...);
		if (pid < 0) {
			create->result = ERR_PTR(pid);
			complete(create->started);
		}
		// else: kthread() will do complete()

		return;

Oleg.

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