linux-kernel - Re: [PATCH 10/11] ptrace: move JOBCTL_TRAPPING wait to wait(2) and ptrace_check

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Date:	Wed, 11 May 2011 18:49:47 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	jan.kratochvil@...hat.com, vda.linux@...glemail.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, indan@....nu
Subject: Re: [PATCH 10/11] ptrace: move JOBCTL_TRAPPING wait to wait(2) and
	ptrace_check_attach()

On 05/08, Tejun Heo wrote:
>
> TRAPPING will also be used to implement end of group stop retrapping,
> which can be initiated by tasks other than tracer.  To allow this,

I didn't read the next patch yet, so I can't undestand/comment the
motivation.

But,

> this patch moves TRAPPING wait from attach completion path to
> operations which are actually affected by the transition - wait(2) and
> following ptrace(2) requests.

You know, I'd wish I could find the serious bugs in this patch. The
code becomes really hairy. -EAGAIN in do_wait() doesn't make it more
simple ;)

> Both wait and ptrace paths are updated to retry the operation after
> TRAPPING wait.  Note that wait_task_stopped() now always grabs siglock
> for ptrace waits.  This can be avoided with "task_stopped_code() ->
> rmb() -> TRAPPING -> rmb() -> task_stopped_code()" sequence

And so far I think this would be better, because it seems we can avoid
the retry logic.

First of all, this patch returns one of the user-visible and undesirable
changes. The tracer know that the task is stopped, attaches, and then it
can see the TASK_RUNNING tracee after ptrace(PTRACE_ATTACH) returns.

I agree, this looks minor. But if we can tolerate this, probably we can
tolerate another oddity: wait_task_stopped() can succeed and eat the
stop code before the tracee actually stopps, no?

IOW, ignoring mb's and read-ordering, suppose that we simply change
task_stopped_code:

		if (ptrace) {
	-		if (task_is_stopped_or_traced(p))
	+		if (task_is_traced(p) || JOBCTL_TRAPPING)
				return &p->exit_code;
		} else {

As for ptrace_check_attach(), it can simply do wait_event(), we
only need to verify the caller is the tracer. No need to play with
lock/unlock/retry.

What do you think?

Oleg.

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