Roland McGrath wrote:
>> I've read through most of the docs (although my eyes certainly glazed
>> over when I got to the tracehook stuff).
>
> That stuff is for kernel maintainers, as it says. You don't really need to
> know. For that part, just checking for typos is all the help it needs.
>
>> What's there seems to be pretty good. I think it is missing (or at
>> least I missed it) a good description of how to asynchronously stop a
>> thread. What are the ins-and-outs here, knowing how/when to use
>> UTRACE_STOP vs. UTRACE_INTERRUPT, etc.
>
> Yeah, the kerneldoc-driven format doesn't lend itself to a lot of
> exposition. The details are in there in the utrace_control description.
> But I guess it's not real obvious how to put it all together.
>
> How is the "Stopping Safely" section? That seems like the place to add
> something more direct about this. Was there something other than the
> UTRACE_INTERRUPT issue that didn't seem clear?
>
> Is that not clear to you, or is it just not clear in the documentation?
Probably both - see below.
> You use UTRACE_INTERRUPT when you want to interfere with system calls in
> progress (or blocking page faults or whatever). To interrupt and stop
> (like PTRACE_ATTACH does) and handle it already being stopped, you really
> want to do UTRACE_STOP first and see 0 if it's already stopped. If it's
> not already stopped, you get -EINPROGRESS and then can do UTRACE_INTERRUPT
> to be sure that you interrupt it and that the race complexity of it being
> stopped before or waking up is minimized. (If you just do UTRACE_INTERRUPT
> first, then you'll get a callback soon--unless it's stopped. Then you
> won't, but if you do UTRACE_STOP second to see if it's stopped, then you
> could get the callback in between and your life gets more hairy.)
Ah.
That paragraph makes lots of sense - I wish it could be included
somewhere. So, basically, you do something like this:
rc = utrace_control(t, engine, UTRACE_STOP);
if (rc == -EINPROGRESS) {
rc = utrace_control(t, engine, UTRACE_INTERRUPT);
}
--
David Smith
[EMAIL PROTECTED]
Red Hat
http://www.redhat.com
256.217.0141 (direct)
256.837.0057 (fax)
