linux-kernel - Re: [PATCH 3/4] kthreads: rework kthread

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Message-Id: <200901312246.07737.rusty@rustcorp.com.au>
Date:	Sat, 31 Jan 2009 22:46:07 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Vitaliy Gusev <vgusev@...nvz.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] kthreads: rework kthread_stop()

On Friday 30 January 2009 23:20:58 Oleg Nesterov wrote:
> On 01/30, Oleg Nesterov wrote:
> >
> > With this patch kthread() allocates all neccesary data (struct kthread)
> > on its own stack, globals kthread_stop_xxx are deleted. ->vfork_done
> > is used as a pointer into "struct kthread", this means kthread_stop()
> > can easily wait for kthread's exit.
> 
> To simplify the review, please see the code with the patch applied.

Hmm, I thought about using the parent-child relationship and fairly normal
wait() semantics, but this looks simpler.

> struct kthread {
> 	int should_stop;
> 	struct completion exited;
> };

Mildly prefer bool in new code.

> #define to_kthread(tsk)	\
> 	container_of((tsk)->vfork_done, struct kthread, exited)

This needs a comment.  Especially since to_xxx(yyy) is usually simply a
container_of(yyy, xxx, member).  This one is special.

> int kthread_stop(struct task_struct *k)
> {
> 	struct kthread *kthread;
> 	int ret;
> 
> 	trace_sched_kthread_stop(k);
> 	get_task_struct(k);
> 
> 	kthread = to_kthread(k);
> 	barrier(); /* it might have exited */
> 	if (k->vfork_done != NULL) {
> 		kthread->should_stop = 1;
> 		wake_up_process(k);
> 		wait_for_completion(&kthread->exited);
> 	}
> 	ret = k->exit_code;

I don't think this works.  How does do_exit() preserve a stack var, other
than for a few cycles longer?  Sure, the vfork_done will be OK, but this code
here will not be.  I think you'd need a get_task_struct(current) before the
do_exit(ret) (the case where the kthread fn calls do_exit() is fine: you're
not allowed to call kthread stop on such threads).

In which case using vfork_done is really just a convenience pointer inside
struct task_struct to stash the struct kthread.  And that's horribly ugly, which is why I stuck with a simple global.  Changing to a linked-list of things to stop would avoid the deadlock you mentioned where a kthread stops another kthread.

Thanks,
Rusty.
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