Re: [PATCH v2] kdb: Fix stack crawling on 'running' CPUs that aren't the master

[Daniel Thompson]

On Mon, Aug 26, 2019 at 03:25:43PM -0700, Doug Anderson wrote:
> Jason / Daniel,
> 
> On Wed, Jul 31, 2019 at 11:38 AM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
> >
> > In kdb when you do 'btc' (back trace on CPU) it doesn't necessarily
> > give you the right info.  Specifically on many architectures
> > (including arm64, where I tested) you can't dump the stack of a
> > "running" process that isn't the process running on the current CPU.
> > This can be seen by this:
> >
> >  echo SOFTLOCKUP > /sys/kernel/debug/provoke-crash/DIRECT
> >  # wait 2 seconds
> >  <sysrq>g
> >
> > Here's what I see now on rk3399-gru-kevin.  I see the stack crawl for
> > the CPU that handled the sysrq but everything else just shows me stuck
> > in __switch_to() which is bogus:
> >
> > ======
> >
> > [0]kdb> btc
> > btc: cpu status: Currently on cpu 0
> > Available cpus: 0, 1-3(I), 4, 5(I)
> > Stack traceback for pid 0
> > 0xffffff801101a9c0        0        0  1    0   R  0xffffff801101b3b0 *swapper/0
> > Call trace:
> >  dump_backtrace+0x0/0x138
> >  ...
> >  kgdb_compiled_brk_fn+0x34/0x44
> >  ...
> >  sysrq_handle_dbg+0x34/0x5c
> > Stack traceback for pid 0
> > 0xffffffc0f175a040        0        0  1    1   I  0xffffffc0f175aa30  swapper/1
> > Call trace:
> >  __switch_to+0x1e4/0x240
> >  0xffffffc0f65616c0
> > Stack traceback for pid 0
> > 0xffffffc0f175d040        0        0  1    2   I  0xffffffc0f175da30  swapper/2
> > Call trace:
> >  __switch_to+0x1e4/0x240
> >  0xffffffc0f65806c0
> > Stack traceback for pid 0
> > 0xffffffc0f175b040        0        0  1    3   I  0xffffffc0f175ba30  swapper/3
> > Call trace:
> >  __switch_to+0x1e4/0x240
> >  0xffffffc0f659f6c0
> > Stack traceback for pid 1474
> > 0xffffffc0dde8b040     1474      727  1    4   R  0xffffffc0dde8ba30  bash
> > Call trace:
> >  __switch_to+0x1e4/0x240
> >  __schedule+0x464/0x618
> >  0xffffffc0dde8b040
> > Stack traceback for pid 0
> > 0xffffffc0f17b0040        0        0  1    5   I  0xffffffc0f17b0a30  swapper/5
> > Call trace:
> >  __switch_to+0x1e4/0x240
> >  0xffffffc0f65dd6c0
> >
> > ===
> >
> > The problem is that 'btc' eventually boils down to
> >   show_stack(task_struct, NULL);
> >
> > ...and show_stack() doesn't work for "running" CPUs because their
> > registers haven't been stashed.
> >
> > On x86 things might work better (I haven't tested) because kdb has a
> > special case for x86 in kdb_show_stack() where it passes the stack
> > pointer to show_stack().  This wouldn't work on arm64 where the stack
> > crawling function seems needs the "fp" and "pc", not the "sp" which is
> > presumably why arm64's show_stack() function totally ignores the "sp"
> > parameter.
> >
> > NOTE: we _can_ get a good stack dump for all the cpus if we manually
> > switch each one to the kdb master and do a back trace.  AKA:
> >   cpu 4
> >   bt
> > ...will give the expected trace.  That's because now arm64's
> > dump_backtrace will now see that "tsk == current" and go through a
> > different path.
> >
> > In this patch I fix the problems by catching a request to stack crawl
> > a task that's running on a CPU and then I ask that CPU to do the stack
> > crawl.
> >
> > NOTE: this will (presumably) change what stack crawls are printed for
> > x86 machines.  Now kdb functions will show up in the stack crawl.
> > Presumably this is OK but if it's not we can go back and add a special
> > case for x86 again.
> >
> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > ---
> >
> > Changes in v2:
> > - Totally new approach; now arch agnostic.
> >
> >  kernel/debug/debug_core.c |  5 +++++
> >  kernel/debug/debug_core.h |  1 +
> >  kernel/debug/kdb/kdb_bt.c | 44 ++++++++++++++++++++++++++++++---------
> >  3 files changed, 40 insertions(+), 10 deletions(-)
> 
> Did either of you have thoughts on this patch?

Hi Doug

Sorry about this. It got backlogged during a recent holiday... it's still
on the list.

I took a quick look a week or so ago but at this point I haven't yet
tested out the behaviour on x86 and I wanted to do a closer review to
check I am happy with the barriering.


Daniel.

> 
> -Doug