Re: [PATCH v2] kdb: Fix stack crawling on 'running' CPUs that aren't the master

From: Doug Anderson
Date: Wed Sep 25 2019 - 16:03:17 EST

Hi,

On Fri, Aug 30, 2019 at 7:52 AM Daniel Thompson
<daniel.thompson@xxxxxxxxxx> wrote:
>
> On Wed, Jul 31, 2019 at 11:37:32AM -0700, Douglas Anderson wrote:
> > In kdb when you do 'btc' (back trace on CPU) it doesn't necessarily
> > give you the right info. Specifically on many architectures
> > (including arm64, where I tested) you can't dump the stack of a
> > "running" process that isn't the process running on the current CPU.
> > This can be seen by this:
> >
> > echo SOFTLOCKUP > /sys/kernel/debug/provoke-crash/DIRECT
> > # wait 2 seconds
> > <sysrq>g
> >
> > Here's what I see now on rk3399-gru-kevin. I see the stack crawl for
> > the CPU that handled the sysrq but everything else just shows me stuck
> > in __switch_to() which is bogus:
> >
> > ======
> >
> > [0]kdb> btc
> > btc: cpu status: Currently on cpu 0
> > Available cpus: 0, 1-3(I), 4, 5(I)
> > Stack traceback for pid 0
> > 0xffffff801101a9c0 0 0 1 0 R 0xffffff801101b3b0 *swapper/0
> > Call trace:
> > dump_backtrace+0x0/0x138
> > ...
> > kgdb_compiled_brk_fn+0x34/0x44
> > ...
> > sysrq_handle_dbg+0x34/0x5c
> > Stack traceback for pid 0
> > 0xffffffc0f175a040 0 0 1 1 I 0xffffffc0f175aa30 swapper/1
> > Call trace:
> > __switch_to+0x1e4/0x240
> > 0xffffffc0f65616c0
> > Stack traceback for pid 0
> > 0xffffffc0f175d040 0 0 1 2 I 0xffffffc0f175da30 swapper/2
> > Call trace:
> > __switch_to+0x1e4/0x240
> > 0xffffffc0f65806c0
> > Stack traceback for pid 0
> > 0xffffffc0f175b040 0 0 1 3 I 0xffffffc0f175ba30 swapper/3
> > Call trace:
> > __switch_to+0x1e4/0x240
> > 0xffffffc0f659f6c0
> > Stack traceback for pid 1474
> > 0xffffffc0dde8b040 1474 727 1 4 R 0xffffffc0dde8ba30 bash
> > Call trace:
> > __switch_to+0x1e4/0x240
> > __schedule+0x464/0x618
> > 0xffffffc0dde8b040
> > Stack traceback for pid 0
> > 0xffffffc0f17b0040 0 0 1 5 I 0xffffffc0f17b0a30 swapper/5
> > Call trace:
> > __switch_to+0x1e4/0x240
> > 0xffffffc0f65dd6c0
> >
> > ===
> >
> > The problem is that 'btc' eventually boils down to
> > show_stack(task_struct, NULL);
> >
> > ...and show_stack() doesn't work for "running" CPUs because their
> > registers haven't been stashed.
> >
> > On x86 things might work better (I haven't tested) because kdb has a
> > special case for x86 in kdb_show_stack() where it passes the stack
> > pointer to show_stack(). This wouldn't work on arm64 where the stack
> > crawling function seems needs the "fp" and "pc", not the "sp" which is
> > presumably why arm64's show_stack() function totally ignores the "sp"
> > parameter.
> >
> > NOTE: we _can_ get a good stack dump for all the cpus if we manually
> > switch each one to the kdb master and do a back trace. AKA:
> > cpu 4
> > bt
> > ...will give the expected trace. That's because now arm64's
> > dump_backtrace will now see that "tsk == current" and go through a
> > different path.
> >
> > In this patch I fix the problems by catching a request to stack crawl
> > a task that's running on a CPU and then I ask that CPU to do the stack
> > crawl.
> >
> > NOTE: this will (presumably) change what stack crawls are printed for
> > x86 machines. Now kdb functions will show up in the stack crawl.
> > Presumably this is OK but if it's not we can go back and add a special
> > case for x86 again.
> >
> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
>
> I think this approach can be made work but there are problems as things
> exist today, see below.
>
>
> > ---
> >
> > Changes in v2:
> > - Totally new approach; now arch agnostic.
> >
> > kernel/debug/debug_core.c | 5 +++++
> > kernel/debug/debug_core.h | 1 +
> > kernel/debug/kdb/kdb_bt.c | 44 ++++++++++++++++++++++++++++++---------
> > 3 files changed, 40 insertions(+), 10 deletions(-)
> >
> > diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
> > index 5cc608de6883..a89c72714fe6 100644
> > --- a/kernel/debug/debug_core.c
> > +++ b/kernel/debug/debug_core.c
> > @@ -92,6 +92,8 @@ static int kgdb_use_con;
> > bool dbg_is_early = true;
> > /* Next cpu to become the master debug core */
> > int dbg_switch_cpu;
> > +/* cpu number of slave we request a stack crawl of */
> > +int dbg_slave_dumpstack_cpu = -1;
> >
> > /* Use kdb or gdbserver mode */
> > int dbg_kdb_mode = 1;
> > @@ -580,6 +582,9 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
> > atomic_xchg(&kgdb_active, cpu);
> > break;
> > }
> > + } else if (dbg_slave_dumpstack_cpu == cpu) {
>
> Couldn't this be encoded in the exception state?

Yup, but it requires exporting the exception state from debug_core.c
(or exporting a function that sets this). Ah, but below you said you
wanted the whole stack crawling on a CPU moved to debug_core.c, so
done.


> > + dump_stack();
> > + dbg_slave_dumpstack_cpu = -1;
>
> > } else if (kgdb_info[cpu].exception_state & DCPU_IS_SLAVE) {
> > if (!raw_spin_is_locked(&dbg_slave_lock))
> > goto return_normal;
> > diff --git a/kernel/debug/debug_core.h b/kernel/debug/debug_core.h
> > index b4a7c326d546..dca74d5caef2 100644
> > --- a/kernel/debug/debug_core.h
> > +++ b/kernel/debug/debug_core.h
> > @@ -62,6 +62,7 @@ extern int dbg_io_get_char(void);
> > /* Switch from one cpu to another */
> > #define DBG_SWITCH_CPU_EVENT -123456
> > extern int dbg_switch_cpu;
> > +extern int dbg_slave_dumpstack_cpu;
> >
> > /* gdbstub interface functions */
> > extern int gdb_serial_stub(struct kgdb_state *ks);
> > diff --git a/kernel/debug/kdb/kdb_bt.c b/kernel/debug/kdb/kdb_bt.c
> > index 7e2379aa0a1e..10095ae05826 100644
> > --- a/kernel/debug/kdb/kdb_bt.c
> > +++ b/kernel/debug/kdb/kdb_bt.c
> > @@ -10,6 +10,7 @@
> > */
> >
> > #include <linux/ctype.h>
> > +#include <linux/delay.h>
> > #include <linux/string.h>
> > #include <linux/kernel.h>
> > #include <linux/sched/signal.h>
> > @@ -22,20 +23,43 @@
> > static void kdb_show_stack(struct task_struct *p, void *addr)
> > {
> > int old_lvl = console_loglevel;
> > + int time_left;
> > + int cpu;
> > +
> > console_loglevel = CONSOLE_LOGLEVEL_MOTORMOUTH;
> > kdb_trap_printk++;
> > - kdb_set_current_task(p);
> > - if (addr) {
> > - show_stack((struct task_struct *)p, addr);
> > - } else if (kdb_current_regs) {
> > -#ifdef CONFIG_X86
> > - show_stack(p, &kdb_current_regs->sp);
> > -#else
> > - show_stack(p, NULL);
> > -#endif
> > +
> > + if (!addr && kdb_task_has_cpu(p)) {
> > + cpu = kdb_process_cpu(p);
> > +
> > + if (cpu == raw_smp_processor_id()) {
> > + dump_stack();
> > + goto exit;
>
> This goto is not for error recovery but looks like it is. Why can't we
> use normal control flow here (extracting the remote stack dump logic
> into a seperate function if the right margin is getting too close)?
>
> In fact to be honest a function call would be useful anyway since I'd
> rather have all the resulting horror in a single file (debug_core.c).

Sure. Done.


> > + }
> > +
> > + /*
> > + * In general architectures don't support dumping the stack
> > + * of a "running" process that's not the current one so if
> > + * we want to dump the stack of a running process that's not
> > + * the master then we'll set a global letting the slave
> > + * (which should be looping) know to dump its own stack.
> > + */
> > + dbg_slave_dumpstack_cpu = cpu;
> > + for (time_left = MSEC_PER_SEC; time_left; time_left--) {
> > + udelay(1000);
> > + if (dbg_slave_dumpstack_cpu == -1)
> > + break;
> > + }
>
> This timeout does not interact correctly with the pager (the timer does
> not get reset when we sit in the pager loop waiting for user to tell us
> to continue).

Nice catch, thanks. Probably the easiest thing to do is to get rid of
this timeout but put in a check to make sure that the CPU has the
"IN_SLAVE" flag set. This was important since you otherwise could get
into this code path by doing "ps" to see what process was running on a
non-rounded-up CPU and then "btp <pid>". v3 should handle this
without the timeout.


-Doug