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

[Doug Anderson]

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?

-Doug