Re: [RFC,PATCH 14/14] utrace core

[Oleg Nesterov <oleg@redhat.com>]

On 12/13, Roland McGrath wrote:
>
> > Its not entirely clear why we can check pending_attach outside of the
> > utrace->lock and not be racy.
>
> I think it can be racy sometimes but that does not matter.
> Maybe Oleg can verify my logic here.  If it's right, he can
> add some comments to make it more clear.
>
> There is only a very limited sort of "timeliness" guarantee about
> getting your callbacks after utrace_attach_task()+utrace_set_events().
> If you know somehow that the task was definitely still in TASK_STOPPED
> or TASK_TRACED after utrace_attach_task() returned, then your engine
> gets all possible callbacks starting from when it resumes.  Otherwise,
> you can use utrace_control() with UTRACE_REPORT to ask to get some
> callback "pretty soon".  The only callback events you are ever 100%
> guaranteed about (after success return from utrace_set_events()) are for
> DEATH and REAP, which have an unconditional lock-and-check before making
> engine callbacks.

Yes, I think this is correct. It is fine to miss ->pending_attach == T,
and in any case the new attacher can come right after the check, even
if it was checked under utrace->lock.

It is important that the tracee can't miss, say, UTRACE_REPORT request
(as you already explained), and every time the tracee clears ->resume
it calls splice_attaching().

> In the stopped cases, there are lots of locks and barriers and things
> after resuming.  (Oleg?)

Every time the tracee resumes after TASK_TRACED it uses utrace->lock
to synchronize with utrace_control/etc, it must see any changes.

Oleg.