Re: ptrace problem with 2.6.25 on Itanium

["stephane eranian" <eranian@googlemail.com>]

Petr,

Thanks for checking, I am pretty sure this is a problem introduced recently.
The only thing related to this that I can think of is the TIF_RESTORE_RSE
and the associated TIF_NOTIFY_RESUME.

When I try the same test on 2.6.25:
   $ ./task_ptrace ~/perfmon/pfmon/tests/forktest 10 10
   creating 10 additional process(es)
   10 iterations
   pidu40 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
   FORK new_pid [7541]
   pidu41 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
   SIGSTOP from [7541]
   pidu41 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
   EXITED [7541]
   pidu42 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>>SIGSTOP from [7542]
   pidu42 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
   EXITED [7542]
   pidu40 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>>FORK new_pid [7542]
   pidu40 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
   pidu40 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
   FORK new_pid [7543]
   pidu43 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
   SIGSTOP from [7543]
   pidu43 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
   EXITED [7543]
   pidu44 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>>SIGSTOP from [7544]
   pidu40 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>>FORK new_pid [7544]

On Thu, Apr 24, 2008 at 2:04 PM, Petr Tesarik <ptesarik@suse.cz> wrote:
>
> On Thu, 2008-04-24 at 12:39 +0200, stephane eranian wrote:
>  > Hello everyone,
>  >
>  > I am running into a new problem with perfmon on Itanium and 2.6.25.
>  >
>  > The pfmon tool is able to monitor across fork(). For that it relies on
>  > ptrace() to receive notifications on fork. This works fine on X86 and 2.6.25
>  > however it is currently broken on IA-64.
>  >
>  > Normally, on fork(), the ptracing parent (here pfmon) receives 2 notifications:
>  >
>  >    1. SIGTRAP with event PTRACE_EVENT_FORK to indicate a new process
>  >        is being created. New pid is extracted via PTRACE_GETEVENTMSG
>  >
>  >    2. SIGSTOP with for new pid indicating that child is ready to
>  > execute its first
>  >        instruction
>  >
>  >
>  > The first message allow the tool to create the data structure to for
>  > new process,
>  > the second marks the point where a perfmon context can actually be attached.
>  >
>  > With 2.6.25 on Itanium, the notifications are received out of order,
>  > i.e., the SIGTOP
>  > first and the FORK notification next. Of course, the tool is confused
>  > because until
>  > it sees the FORK event, it does not know the new process.
>  >
>  > This situation never happens on X86 with the same kernel.
>  >
>  > To demonstrate the problem, I have attached a simple test program. You need
>  > to pass the name of a command that creates child processes. Look at the order
>  > between the FORK and SIGSTOP notifications. There is a forktest program in
>  > pfmon/tests.
>  >
>  > I don't have time to track this down. However, I am highly suspicious of this
>  > new TIF_RESTORE_RSE and the arch_ptrace_stop_needed() code. The do_fork()
>  > routine does indeed set SIGSTOP, before it call ptrace_notify(). But this does
>  > not impact X86, which, by the way, does not define arch_ptrace_stop_needed().
>  > I don't have an older kernel handy to run the test. Hopefully someone
>  > on this list
>  > will try this on 2.6.24 or older.
>
>  I tried it on SLES10, which is basically a 2.6.16 with a simplified
>  version of the patch (one which only uses arch_ptrace_stop, but not
>  TIF_RESTORE_RSE) and it works as expected:
>
>  glass:~/ptrace-wrong-notify # ./task_ptrace_attach ./forktest 10 10
>  creating 10 additional process(es)
>  10 iterations
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>  FORK new_pid [6199]
>  pida99 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>  SIGSTOP from [6199]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>  FORK new_pid [6200]
>  pidb00 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>  SIGSTOP from [6200]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>  FORK new_pid [6201]
>  pida99 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6199]
>  pidb00 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6200]
>  pidb01 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>  SIGSTOP from [6201]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
>  pidb01 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6201]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>  FORK new_pid [6202]
>  pidb02 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>  SIGSTOP from [6202]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>  FORK new_pid [6203]
>  pidb02 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6202]
>  pidb03 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>  SIGSTOP from [6203]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
>  pidb03 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6203]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>  FORK new_pid [6204]
>  pidb04 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>  SIGSTOP from [6204]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>  FORK new_pid [6205]
>  pidb04 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6204]
>  pidb05 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>  SIGSTOP from [6205]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>  FORK new_pid [6206]
>  pidb05 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6205]
>  pidb06 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>  SIGSTOP from [6206]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>  FORK new_pid [6207]
>  pidb06 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6206]
>  pidb07 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>  SIGSTOP from [6207]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig=5
>  FORK new_pid [6208]
>  pidb07 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6207]
>  pidb08 errno=0 exited=0 stopped=1 signaled=0 stopsig\x19
>  SIGSTOP from [6208]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
>  pidb08 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6208]
>  pida98 errno=0 exited=0 stopped=1 signaled=0 stopsig\x17
>  pida98 errno=0 exited=1 stopped=0 signaled=0 stopsig=0
>  EXITED [6198]
>
>  So, if something is broken, it must be the TIF_RESTORE_RSE part of the
>  patch, or an unexpected side effect of switching to the generic
>  sys_ptrace. I plan to have a look at mainline later today...
>
>  Kind regards,
>  Petr Tesarik
>
>