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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@chromium.org> Acked-by: Will Deacon <will@kernel.org> Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org> |
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| .gitignore | ||
| kdb_bp.c | ||
| kdb_bt.c | ||
| kdb_cmds | ||
| kdb_debugger.c | ||
| kdb_io.c | ||
| kdb_keyboard.c | ||
| kdb_main.c | ||
| kdb_private.h | ||
| kdb_support.c | ||
| Makefile | ||