linux-kernel - Re: [PATCH] powerpc/xmon: really enable xmon when a breakpoint is set

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Date:   Tue, 22 May 2018 12:53:53 +0530
From:   Vaibhav Jain <vaibhav@...ux.vnet.ibm.com>
To:     Michal Suchanek <msuchanek@...e.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Balbir Singh <bsingharora@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Breno Leitao <leitao@...ian.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        "Guilherme G. Piccoli" <gpiccoli@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Cc:     Michal Suchanek <msuchanek@...e.de>
Subject: Re: [PATCH] powerpc/xmon: really enable xmon when a breakpoint is set

Thanks for the patch Michal,

Michal Suchanek <msuchanek@...e.de> 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 <vaibhav@...ux.vnet.ibm.com>
Linux Technology Center, IBM India Pvt. Ltd.