Re: Q: mutlithreaded tracees && clone/exit

[Oleg Nesterov <oleg at redhat dot com>]

On 07/16, Roland McGrath wrote:
>
> > Q1: if gsbstub reported that the tracee has exited (say, we sent
> > '$X09#..' to gdb), can gsbstub assume it can forget about this thread?
>
> I think so.  In the ptrace-based implementation of gdbserver, it sends
> X or W reports after it has done wait and gotten a death status.  In
> Linux, that means the zombie is reaped and its PID is available for reuse.

Yes, but this doesn't necessarily mean gdbserver can forget its exit
code (or some internal state), I do not see anything about this in docs.

But yes, I think it can too.

> > The main question is, I do not understand how gdbstub should handle the
> > multithreaded targets.
>
> It's not really clear to me when gdb decides to ask for the thread
> list.

Never in my (limited) testing.

> It looks like it only does it at extended-remote attach time if
> you have set non-stop mode.

OK, I'll check this. But this doesn't really matter.

> > Q2: Shouldn't gdbstub let debugger know about sub-threads somehow?
>
> That's what I would expect in the abstract.  But I know that gdb
> didn't used to get new-thread notifications from ptrace either.  It
> looks like the linux-nat code does track PTRACE_EVENT_CLONE now.
> But it may be that the gdbserver code and remote protocol were made
> to match how things were when the native ptrace case didn't do that.

gdbserver tracks PTRACE_EVENT_CLONE, yes. But it doesn't inform gdb.

> gdb also uses higher-level knowledge read from user memory
> (libthread_db) for some aspects of thread tracking.

Well, yes and no (if I understood your message correctly).

I have already looked at this code in horror. I really hope this magic
is not needed for our purposes.

It is gdbserver, not gdb, who uses libthread_db to find sub-threads and
do other things.

gdbserver asks gdb what is the symbol's address (say, _thread_db_list_t_next)
via 'qSymbol'.

> > I am asking because that I thought that gdb+gdbserver should
> > try to work the same way as it works without gdbserver, and
> > thus it should see clone/exit.
>
> I agree that's how it seems it should be.

OK, so far it is not clear to me what should we do. If nothing else,
I can replicate the gdbserver's behaviour. But imho it makes sense to
do something more clever.

However, there is the complication I already mentioned. If the main
thread exits, this confuses gdbserver at least. It sends the "$T05"
packets to gdb, then eventually gdb does vCont;c:pTGID.-1 and gdbserver
doesn't work. It doesn't resume sub-threads, doesn't react to ^C, etc.

I guess, gdbserver shouldn't send '$W' packet in this case, this can
confuse gdb (but I didn't verify this yet). OTOH, it is not clear if
gdbserver can delay this notification until all threads exit. Say,
what should gdbserver do if gdb sends a private signal to the exited
main thread? Or do something else which assumes it alive.

Let's see what other experts think...

Oleg.