Messages in this thread |  | | | Subject | Re: [patch 3/14] x86, ptrace, bts: stop bts tracing early in do_exit | | From | Markus Metzger <> | | Date | Fri, 27 Mar 2009 20:25:51 +0100 |
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On Fri, 2009-03-27 at 18:24 +0100, Oleg Nesterov wrote: > On 03/27, Metzger, Markus T wrote: > > > > >-----Original Message----- > > >From: Oleg Nesterov [mailto:oleg@redhat.com] > > >Sent: Friday, March 27, 2009 3:35 PM > > > > >> +static void ptrace_bts_exit(void) > > >> +{ > > >> + if (unlikely(!list_empty(¤t->ptraced))) > > >> + ptrace_bts_exit_tracer(); > > >> + > > >> + if (unlikely(current->bts)) > > >> + ptrace_bts_exit_tracee(); > > >> +} > > > > > >Could you explain why do we need ptrace_bts_exit_tracee() ? > > > > > >If current is traced, the tracer should do ptrace_bts_release() > > >eventually, no? > > > > If current is traced and exits, it may be reaped by another thread that is not > > the tracer (that's actually your example you made in an earlier thread to > > describe the race between a normal detach and an exiting tracee). > > > > The ptrace_unlink() call to detach the tracer is executed with irq's disabled. > > I need irq's enabled (see the other discussion, to wait for the traced task). > > OK, > > > Therefore, I have the tracee disable bts tracing itself when it exits. > > > > > > >And if we really need to do ptrace_bts_exit_tracee(), then > > >"if (unlikely(current->bts))" check is racy. The tracer > > >can do PTRACE_BTS_CONFIG right after the check. > > > > The ptrace system call to do this would require the tracee to be stopped. > > Yes, but this doesn't matter. > > The tracer starts ptrace(PTRACE_BTS_CONFIG) and stops the tracee. > But when the tracer calls ptrace_bts_config() the tracee can be already > killed, and it can exit and bypass ptrace_bts_exit_tracee().
Thanks for your review! This is really helpful.
This exit race is a nasty thing. I think I need to either prevent a task from running while I change the tracing configuration, or have the tracee execute all the setup/cleanup code.
The former looks more attractive to me. I would expect that there is some infrastructure for that. I asked for pointers in a previous email, already.
thanks and regards, markus.
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