On Tue, 12 May 2020 at 19:55, Daniel Thompson
<daniel.thomp...@linaro.org> 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.g...@linaro.org> > > --- > > > > 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 > >
