Re: [PATCH] kgdb: Fix broken handling of printk() in NMI context

[Sumit Garg]

On Tue, 12 May 2020 at 19:55, Daniel Thompson
<daniel.thompson@xxxxxxxxxx> wrote:
>
> On Tue, May 12, 2020 at 02:18:34PM +0530, Sumit Garg wrote:
> > Since commit 42a0bb3f7138 ("printk/nmi: generic solution for safe printk
> > in NMI"), kgdb entry in NMI context defaults to use safe NMI printk()
>
> I didn't see the author on Cc: nor any of the folks whose hands it
> passed through. It would definitely be good to involve them in this
> discussion.
>

Thanks for updating the Cc: list.

>
> > which involves CPU specific buffers and deferred printk() until exit from
> > NMI context.
> >
> > But kgdb being a stop-the-world debugger, we don't want to defer printk()
> > especially backtrace on corresponding CPUs. So instead switch to normal
> > printk() mode in kgdb_cpu_enter() if entry is in NMI context.
>
> So, firstly I should *definitely* take a mea cupla for not shouting
> about this at the time (I was on Cc:... twice). Only thing I can say
> confidently is that the test suite didn't yell about this and so I
> didn't look at this as closely as I should have done (and that it
> didn't yell is mostly because I'm still building out the test suite
> coverage).
>
> Anyhow...
>
> This feels a little like we are smearing the printk() interception logic
> across the kernel in ways that make things hard to read. If we accepted
> this patch we then have, the new NMI interception logic, the old kdb
> interception logic and some hacks in the kgdb trap handler to defang the
> NMI interception logic and force the kdb logic to kick in.
>
> Wouldn't it be better to migrate kdb interception logic up a couple of
> levels so that it continues to function even when we are in nmi printk
> mode. That way *all* the printk() interception code would end up in
> one place.
>

Yes it would be better to have all printk() interception code at one
place. Let me see if I can come up with an integrated logic.

> Finally some clue description of how to provoke the problem would be
> useful... that sort of things helps me to grow the test suite coverage.
>

Sure I will update the description. BTW, this issue can be easily
reproduced via issuing a backtrace (kdb command: "bt or btc") on a CPU
which entered kgdb in NMI context.

-Sumit

>
> Daniel.
>
>
> >
> > Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> > ---
> >
> > Similar change was posted earlier specific to arm64 here [1]. But after
> > discussions it emerged out that this broken handling of printk() in NMI
> > context should be a common problem that is relevant to other archs as well.
> > So fix this handling in kgdb_cpu_enter() as there can be multiple entry
> > points to kgdb in NMI context.
> >
> > [1] https://lkml.org/lkml/2020/4/24/328
> >
> >  kernel/debug/debug_core.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
> > index 2b7c9b6..ab2933f 100644
> > --- a/kernel/debug/debug_core.c
> > +++ b/kernel/debug/debug_core.c
> > @@ -567,6 +567,15 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
> >       kgdb_info[ks->cpu].enter_kgdb++;
> >       kgdb_info[ks->cpu].exception_state |= exception_state;
> >
> > +     /*
> > +      * kgdb entry in NMI context defaults to use safe NMI printk() which
> > +      * involves CPU specific buffers and deferred printk() until exit from
> > +      * NMI context. But kgdb being a stop-the-world debugger, we don't want
> > +      * to defer printk(). So instead switch to normal printk() mode here.
> > +      */
> > +     if (in_nmi())
> > +             printk_nmi_exit();
> > +
> >       if (exception_state == DCPU_WANT_MASTER)
> >               atomic_inc(&masters_in_kgdb);
> >       else
> > @@ -635,6 +644,8 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
> >                       atomic_dec(&slaves_in_kgdb);
> >                       dbg_touch_watchdogs();
> >                       local_irq_restore(flags);
> > +                     if (in_nmi())
> > +                             printk_nmi_enter();
> >                       return 0;
> >               }
> >               cpu_relax();
> > @@ -772,6 +783,8 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
> >       raw_spin_unlock(&dbg_master_lock);
> >       dbg_touch_watchdogs();
> >       local_irq_restore(flags);
> > +     if (in_nmi())
> > +             printk_nmi_enter();
> >
> >       return kgdb_info[cpu].ret_state;
> >  }
> > --
> > 2.7.4
> >