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 <diand...@chromium.org> > 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 <diand...@chromium.org> > > --- > > > > 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 _______________________________________________ Kgdb-bugreport mailing list Kgdb-bugreport@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kgdb-bugreport
