Re: kernel read fault when accessing context variables

[Mark Wielaard <mjw at redhat dot com>]

On Thu, 2010-07-08 at 09:15 -0700, tonyg362 wrote:
> Could you tell me if it works for you if you look at a line in sched.c where
> prev is set (or near it), and not just at the beginning of the function? In
> the meantime I will try using a gcc version as suggested by Frank. As for my
> compiler I used 4.4.1 which I guess is the latest version available through
> apt-get on Ubuntu 9.10. I would like to know if anybody can get this working
> on Ubuntu 9.10 so I can know their setup.

I looked a bit at the sources, and when I set the probe at line 42 of
the function (if (likely(prev != next))) it seems to work fine:

$ stap -e 'probe kernel.statement("schedule@kernel/sched.c+42")
  { ppid = $prev->tgid; npid = $next->tgid;
    printf("ppid:%d, npid:%d\n", ppid, npid) }'
ppid:24737, npid:0
ppid:0, npid:21431
ppid:21431, npid:23130
ppid:23130, npid:22989
ppid:22989, npid:21431
[...]

If I set the probe earlier next isn't assigned yet in the function and I
see what you are seeing:

$ stap -e 'probe kernel.statement("schedule@kernel/sched.c+16")
  { ppid = $prev->tgid; npid = $next->tgid;
    printf("ppid:%d, npid:%d\n", ppid, npid) }'
ERROR: kernel read fault at 0x0000129c29e10ec5 (addr) near identifier
'$next' at <input>:1:83

So it might actually be that in your case the probe is at a point where
next isn't initialized yet?

Cheers,

Mark