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

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Date:   Wed, 23 May 2018 19:58:28 +0200
From:   Michal Suchánek <msuchanek@...e.de>
To:     Vaibhav Jain <vaibhav@...ux.vnet.ibm.com>
Cc:     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
Subject: Re: [PATCH] powerpc/xmon: really enable xmon when a breakpoint is
 set

On Tue, 22 May 2018 12:53:53 +0530
Vaibhav Jain <vaibhav@...ux.vnet.ibm.com> wrote:

> 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.

Indeed, setting xmon_on is sufficient. Will resend the patch.

Thanks

Michal

> 
> > 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.
> 
>