From: Oleg Nesterov <oleg@redhat.com>
To: Roland McGrath <roland@redhat.com>
Cc: archer@sourceware.org
Subject: Re: Q: mutlithreaded tracees && clone/exit
Date: Sun, 18 Jul 2010 17:51:00 -0000 [thread overview]
Message-ID: <20100718174851.GA15528@redhat.com> (raw)
In-Reply-To: <20100716213950.7A7BC40B45@magilla.sf.frob.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.
next prev parent reply other threads:[~2010-07-18 17:51 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-07-16 20:54 Oleg Nesterov
2010-07-16 21:40 ` Roland McGrath
2010-07-18 17:51 ` Oleg Nesterov [this message]
2010-07-18 18:04 ` Oleg Nesterov
2010-07-18 20:22 ` Roland McGrath
2010-07-19 13:44 ` BUG: add_thread_silent()->switch_to_thread(minus_one_ptid) is wrong Oleg Nesterov
2010-07-19 15:36 ` Oleg Nesterov
2010-07-19 16:01 ` Q: mutlithreaded tracees && clone/exit Jan Kratochvil
2010-07-19 22:57 ` Roland McGrath
2010-07-20 13:18 ` Oleg Nesterov
2010-07-20 14:04 ` BUG? gdb, non-stop && c -a Oleg Nesterov
2010-07-20 14:12 ` Jan Kratochvil
2010-07-20 14:49 ` Oleg Nesterov
2010-07-20 15:08 ` Jan Kratochvil
2010-07-20 15:28 ` Oleg Nesterov
2010-07-20 19:43 ` Roland McGrath
2010-07-21 7:59 ` Oleg Nesterov
2010-07-21 8:10 ` Jan Kratochvil
2010-07-21 11:12 ` Oleg Nesterov
2010-07-20 14:21 ` Q: mutlithreaded tracees && clone/exit Jan Kratochvil
2010-07-20 15:09 ` Oleg Nesterov
2010-07-20 19:41 ` Roland McGrath
2010-07-21 8:32 ` Oleg Nesterov
2010-07-20 14:43 ` Q: who maintains the STOPPED/RUNNING state? Oleg Nesterov
[not found] ` <y0mk4ophmvn.fsf@fche.csb>
2010-07-21 10:20 ` Oleg Nesterov
2010-07-21 10:51 ` Oleg Nesterov
2010-07-21 17:06 ` Q: multiple inferiors, all-stop && vCont Oleg Nesterov
2010-07-21 20:42 ` Roland McGrath
2010-07-23 17:33 ` Oleg Nesterov
2010-07-26 14:30 ` Oleg Nesterov
2010-07-26 16:06 ` Oleg Nesterov
2010-07-28 18:19 ` gdbstub initial code, another approach Oleg Nesterov
2010-07-29 21:38 ` Frank Ch. Eigler
2010-07-30 13:00 ` Oleg Nesterov
2010-07-30 13:16 ` Frank Ch. Eigler
2010-07-30 15:01 ` Oleg Nesterov
2010-07-30 13:25 ` Jan Kratochvil
2010-07-30 14:44 ` Oleg Nesterov
2010-07-30 15:20 ` Jan Kratochvil
2010-08-02 12:54 ` Oleg Nesterov
2010-08-03 13:55 ` Jan Kratochvil
2010-07-30 17:59 ` Tom Tromey
2010-08-02 18:25 ` Oleg Nesterov
2010-08-02 23:54 ` Jan Kratochvil
2010-08-03 12:27 ` Q: %Stop && gdb crash Oleg Nesterov
2010-08-03 13:17 ` Oleg Nesterov
2010-08-03 19:57 ` Kevin Buettner
2010-08-04 19:42 ` Oleg Nesterov
2010-08-04 23:32 ` Kevin Buettner
2010-08-05 18:24 ` Oleg Nesterov
2010-08-03 13:36 ` Jan Kratochvil
2010-08-03 15:09 ` Oleg Nesterov
2010-08-03 12:39 ` Q: multiple inferiors, all-stop && vCont Jan Kratochvil
2010-08-03 14:32 ` Oleg Nesterov
2010-08-03 15:55 ` Jan Kratochvil
2010-08-03 16:56 ` Oleg Nesterov
2010-08-03 18:37 ` Jan Kratochvil
2010-08-18 17:07 ` Jan Kratochvil
2010-08-18 19:22 ` Roland McGrath
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