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Date: Sat, 19 Feb 2011 20:37:00 -0000
From: Oleg Nesterov <oleg@redhat.com>
To: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: Roland McGrath <roland@redhat.com>, Project Archer <archer@sourceware.org>
Subject: Re: ptrace improvement: PTRACE_O_INHERIT
Message-ID: <20110219202921.GA12280@redhat.com>
References: <20110214193052.3EC8D1814BA@magilla.sf.frob.com> <20110214193812.GA20765@redhat.com> <20110215003551.BC1EA1802A2@magilla.sf.frob.com> <20110215130805.GA30742@redhat.com> <20110215214333.GA18086@host1.dyn.jankratochvil.net> <20110215215634.2665A1806E0@magilla.sf.frob.com> <20110216193356.GB15576@redhat.com> <20110216194534.2D39018020E@magilla.sf.frob.com> <20110216200110.GA17601@redhat.com> <20110219194758.GA8994@host1.dyn.jankratochvil.net>
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On 02/19, Jan Kratochvil wrote:
>
> On Wed, 16 Feb 2011 21:01:10 +0100, Oleg Nesterov wrote:
> > On 02/16, Roland McGrath wrote:
> > > If the process-wide PID (tgid) is not reused
> > > ...
> > > then tgkill does protect completely against TID reuse.
> >
> > How? sooner or later the process which creates/destroys a thread
> > in a loop will reuse some pid number.
>
> (a) GDB tracks separately  threads (libpthread-managed) and LWPs (TIDs).
>     (So the libthread_db TD_DEATH notification is offtopic for this mail.)
>
> (b) For LWPs it does not use PTRACE_O_TRACEEXIT.
>     /* Do not enable PTRACE_O_TRACEEXIT until GDB is more prepared to support
>        read-only process state.  */
>     (I do not know more regarding this comment.)
>
> Therefore it occasionally uses tkill (TID, 0) instead of PTRACE_O_TRACEEXIT.

Cough. I still can't understand why gdb needs tkill(TID, 0). Please
ignore, I know nothing about gdb implementation.

However. With or without PTRACE_O_TRACEEXIT, the thread can't go away
until ptracer does do_wait(WEXITED). And until it does do_wait() tkill()
succeeds even if the tracee is dead/zombie.

Oleg.