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SubjectRe: [PATCH] powerpc/xmon: really enable xmon when a breakpoint is set
Thanks for the patch Michal,

Michal Suchanek <> writes:

> When single-stepping kernel code from xmon without a debug hook enabled
> the kernel crashes. This can happen when kernel starts with xmon on
> crash disabled but xmon is entered using sysrq.
> Commit e1368d0c9edb ("powerpc/xmon: Setup debugger hooks when first
> break-point is set") adds force_enable_xmon function that prints
> "xmon: Enabling debugger hooks" but does not enable them.
Debugger hooks are enabled just befores debugger() is entered from
sysrq_handle_xmon(). Thats why force_enable_xmon() simply sets sets
'xmon_on=1' and exits.

The problem you are seeing is probably due to sysrq_handle_xmon()
clearing the debugger hooks on return from debugger() as xmon_on was
never set for the 's' xmon command.

> Add the call to xmon_init to install the debugger hooks in
> force_enable_xmon and also call force_enable_xmon when single-stepping
> in xmon.
Only calling force_enable_xmon() from do_step() should be suffice as on exit
from the debugger() the value of xmon_on is checked and if required the
debugger hooks are kept instead of getting cleared.

> arch/powerpc/xmon/xmon.c | 5 +++++
> 1 file changed, 5 insertions(+)
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index a0842f1ff72c..504bd1c3d8b0 100644
> --- a/arch/powerpc/xmon/xmon.c
> +++ b/arch/powerpc/xmon/xmon.c

> @@ -1275,6 +1279,7 @@ static inline void force_enable_xmon(void)
> if (!xmon_on) {
> printf("xmon: Enabling debugger hooks\n");
> xmon_on = 1;
> + xmon_init(1);
> }
> }

As mentioned above call to force_enable_xmon() is usually done in context of
sysrq_handle_xmon() which sets the debugger hooks as soon as its
entered. So I think that this hunk is not really needed.

Vaibhav Jain <>
Linux Technology Center, IBM India Pvt. Ltd.

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