[PATCH v2] arm: kgdb: Fix registers on sleeping tasks

From: Douglas Anderson
Date: Wed Aug 26 2015 - 12:21:03 EST

From: Doug Anderson <dianders@xxxxxxxxxxxx>

Dumping registers from other sleeping tasks in KGDB was totally
failing for me. All registers were reported as 0 in many cases.

The code was using task_pt_regs(task) to try to get other thread
registers. This doesn't appear to be the right place to look. From
my tests, I saw non-zero values in this structure when we were looking
at a kernel thread that had a userspace task associated with it, but
it contained the register values from the userspace task. So even in
the cases where registers weren't reported as 0 we were still not
showing the right thing.

Instead of using task_pt_regs(task) let's use task_thread_info(task).
This is the same place that is referred to when doing a dump of all
sleeping task stacks (kdb_show_stack() -> show_stack() ->
dump_backtrace() -> unwind_backtrace() -> thread_saved_sp()).

As further evidence that this is the right thing to do, you can find
the following comment in "gdbstub.c" right before it calls
Pull stuff saved during switch_to; nothing else is accessible (or
even particularly relevant). This should be enough for a stack
...and if you look at switch_to() it only saves r4-r11, sp and lr.
Those are the same registers that I'm getting out of the

With this change you can use "info thread" to see all tasks in the
kernel and you can switch to other tasks and examine them in gdb.

Signed-off-by: Doug Anderson <dianders@xxxxxxxxxxxx>
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
Changes in v2:
- Don't use task_pt_regs(task) at all.

arch/arm/kernel/kgdb.c | 31 ++++++++++++-------------------
1 file changed, 12 insertions(+), 19 deletions(-)

diff --git a/arch/arm/kernel/kgdb.c b/arch/arm/kernel/kgdb.c
index a6ad93c..abb17b0 100644
--- a/arch/arm/kernel/kgdb.c
+++ b/arch/arm/kernel/kgdb.c
@@ -74,7 +74,7 @@ int dbg_set_reg(int regno, void *mem, struct pt_regs *regs)
sleeping_thread_to_gdb_regs(unsigned long *gdb_regs, struct task_struct *task)
- struct pt_regs *thread_regs;
+ struct thread_info *ti;
int regno;

/* Just making sure... */
@@ -86,24 +86,17 @@ sleeping_thread_to_gdb_regs(unsigned long *gdb_regs, struct task_struct *task)
gdb_regs[regno] = 0;

/* Otherwise, we have only some registers from switch_to() */
- thread_regs = task_pt_regs(task);
- gdb_regs[_R0] = thread_regs->ARM_r0;
- gdb_regs[_R1] = thread_regs->ARM_r1;
- gdb_regs[_R2] = thread_regs->ARM_r2;
- gdb_regs[_R3] = thread_regs->ARM_r3;
- gdb_regs[_R4] = thread_regs->ARM_r4;
- gdb_regs[_R5] = thread_regs->ARM_r5;
- gdb_regs[_R6] = thread_regs->ARM_r6;
- gdb_regs[_R7] = thread_regs->ARM_r7;
- gdb_regs[_R8] = thread_regs->ARM_r8;
- gdb_regs[_R9] = thread_regs->ARM_r9;
- gdb_regs[_R10] = thread_regs->ARM_r10;
- gdb_regs[_FP] = thread_regs->ARM_fp;
- gdb_regs[_IP] = thread_regs->ARM_ip;
- gdb_regs[_SPT] = thread_regs->ARM_sp;
- gdb_regs[_LR] = thread_regs->ARM_lr;
- gdb_regs[_PC] = thread_regs->ARM_pc;
- gdb_regs[_CPSR] = thread_regs->ARM_cpsr;
+ ti = task_thread_info(task);
+ gdb_regs[_R4] = ti->cpu_context.r4;
+ gdb_regs[_R5] = ti->cpu_context.r5;
+ gdb_regs[_R6] = ti->cpu_context.r6;
+ gdb_regs[_R7] = ti->cpu_context.r7;
+ gdb_regs[_R8] = ti->cpu_context.r8;
+ gdb_regs[_R9] = ti->cpu_context.r9;
+ gdb_regs[_R10] = ti->cpu_context.sl;
+ gdb_regs[_FP] = ti->cpu_context.fp;
+ gdb_regs[_SPT] = ti->cpu_context.sp;
+ gdb_regs[_PC] = ti->cpu_context.pc;

void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long pc)

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