Messages in this thread | | | Date | Wed, 11 May 2011 21:53:33 +0200 | From | Tejun Heo <> | Subject | Re: [PATCH 10/11] ptrace: move JOBCTL_TRAPPING wait to wait(2) and ptrace_check_attach() |
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Hello,
On Wed, May 11, 2011 at 06:49:47PM +0200, Oleg Nesterov wrote: > On 05/08, Tejun Heo wrote: > > 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 ;)
I don't know. Why is retrying hairy? The whole waiting logic is built for clean retries. The suggested change just does it without intervening sleeping and waking up. I don't see anything particularly hairy there.
> > 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.
Well, the above memory barrier dance wouldn't really change whether retry logic is required or not and I'd _really_ like to avoid complex barrier dances. Even the typical write-B wmb() write-A / read-A rmb() read-B barriers often confuse people. I don't wanna throw in stacked wmb()/rmb() pairs there even if that means an extra locking for ptrace waits.
> 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.
Yes, it does. Sorry about forgetting to mention it in the patch description. I believe this is something we can swallow.
> 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?
Hmmm... interesting. Yeah, the state is visible only through wait(2) and ptrace(2) and for wait(2) TRAPPING is as good as STOPPED/TRACED and we can wait all we want in ptrace_check_attach(). I'll think more about it but seems like a nice idea.
Thank you.
-- tejun
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