Re: [PATCH 10/11] ptrace: move JOBCTL_TRAPPING wait to wait(2) andptrace_check_attach()
From: Oleg Nesterov
Date: Wed May 11 2011 - 13:42:01 EST
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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