[PATCH 0/2] powerpc: oops fixes

[PATCH 0/2] powerpc: oops fixes

[Nicholas Piggin]

Hi,

Couple of small things I've come across. After this, injecting nmi to
enter xmon (via qemu) and then recovering works nicely.

Thanks,
Nick

Nicholas Piggin (2):
  powerpc: fix graceful debugger recovery
  powerpc: fix second nested oops hang

 arch/powerpc/kernel/traps.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

-- 
2.10.2

[PATCH 1/2] powerpc: fix graceful debugger recovery

[Nicholas Piggin]

When exiting xmon with 'x' (exit and recover), oops_begin bails
out immediately, but die then calls __die() and oops_end(), which
cause a lot of bad things to happen.

If the debugger was attached then went to graceful recovery, exit
from die() immediately.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kernel/traps.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 7480902..26c3ba4 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -122,9 +122,6 @@ static unsigned long oops_begin(struct pt_regs *regs)
 	int cpu;
 	unsigned long flags;
 
-	if (debugger(regs))
-		return 1;
-
 	oops_enter();
 
 	/* racy, but better than risking deadlock. */
@@ -227,8 +224,12 @@ NOKPROBE_SYMBOL(__die);
 
 void die(const char *str, struct pt_regs *regs, long err)
 {
-	unsigned long flags = oops_begin(regs);
+	unsigned long flags;
+
+	if (debugger(regs))
+		return;
 
+	flags = oops_begin(regs);
 	if (__die(str, regs, err))
 		err = 0;
 	oops_end(flags, regs, err);
-- 
2.10.2

Re: [PATCH 1/2] powerpc: fix graceful debugger recovery

[Michael Ellerman]

Nicholas Piggin <npiggin@gmail.com> writes:

> When exiting xmon with 'x' (exit and recover), oops_begin bails
> out immediately, but die then calls __die() and oops_end(), which
> cause a lot of bad things to happen.

In fact oops_begin() returns 1, which oops_end() then passes directly to
raw_local_irq_restore() as flags. On 64-bit that actually works because
arch_local_irq_restore() takes just "en" (enable), not real flags. But
on 32-bit it's supposed to be the MSR value. So that's impressively
broken.

> If the debugger was attached then went to graceful recovery, exit
> from die() immediately.

Right. Crucially it doesn't change anything in terms of the actual logic
of oops_begin(), ie. previously oops_begin() did nothing prior to
calling debugger(), and after this patch that remains the same (which
you did mention above but just spelling it out for myself).

cheers

Re: [PATCH 1/2] powerpc: fix graceful debugger recovery

[Nicholas Piggin]

On Thu, 10 Nov 2016 12:35:59 +1100
Michael Ellerman <mpe@ellerman.id.au> wrote:

> Nicholas Piggin <npiggin@gmail.com> writes:
> 
> > When exiting xmon with 'x' (exit and recover), oops_begin bails
> > out immediately, but die then calls __die() and oops_end(), which
> > cause a lot of bad things to happen.  
> 
> In fact oops_begin() returns 1, which oops_end() then passes directly to
> raw_local_irq_restore() as flags. On 64-bit that actually works because
> arch_local_irq_restore() takes just "en" (enable), not real flags. But
> on 32-bit it's supposed to be the MSR value. So that's impressively
> broken.

Yeah, I guess most of the time you either go to debugger with
sysrq, or in case of a crash don't try to graceful recover.

When sending debug NMIs down via system reset it becomes a problem!

> 
> > If the debugger was attached then went to graceful recovery, exit
> > from die() immediately.  
> 
> Right. Crucially it doesn't change anything in terms of the actual logic
> of oops_begin(), ie. previously oops_begin() did nothing prior to
> calling debugger(), and after this patch that remains the same (which
> you did mention above but just spelling it out for myself).

Right.

Thanks,
Nick

Re: [PATCH 1/2] powerpc: fix graceful debugger recovery

[Michael Ellerman]

Nicholas Piggin <npiggin@gmail.com> writes:

> On Thu, 10 Nov 2016 12:35:59 +1100
> Michael Ellerman <mpe@ellerman.id.au> wrote:
>
>> Nicholas Piggin <npiggin@gmail.com> writes:
>> 
>> > When exiting xmon with 'x' (exit and recover), oops_begin bails
>> > out immediately, but die then calls __die() and oops_end(), which
>> > cause a lot of bad things to happen.  
>> 
>> In fact oops_begin() returns 1, which oops_end() then passes directly to
>> raw_local_irq_restore() as flags. On 64-bit that actually works because
>> arch_local_irq_restore() takes just "en" (enable), not real flags. But
>> on 32-bit it's supposed to be the MSR value. So that's impressively
>> broken.
>
> Yeah, I guess most of the time you either go to debugger with
> sysrq, or in case of a crash don't try to graceful recover.

Yeah. It's debatable whether we should even allow graceful recovery, but
it's useful sometimes and regular users probably shouldn't have a
debugger enabled anyway.

cheers

Re: [1/2] powerpc: fix graceful debugger recovery

[Michael Ellerman]

On Tue, 2016-11-08 at 12:14:44 UTC, Nicholas Piggin wrote:
> When exiting xmon with 'x' (exit and recover), oops_begin bails
> out immediately, but die then calls __die() and oops_end(), which
> cause a lot of bad things to happen.
> 
> If the debugger was attached then went to graceful recovery, exit
> from die() immediately.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/6f44b20ee9b4130345c189c0c90ef6

cheers

[PATCH 2/2] powerpc: fix second nested oops hang

[Nicholas Piggin]

When ending an oops, don't clear die_owner unless the nest count
went to zero. This prevents a second nested oops from hanging forever
on the die_lock.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kernel/traps.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 26c3ba4..ba78e38 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -147,14 +147,15 @@ static void oops_end(unsigned long flags, struct pt_regs *regs,
 			       int signr)
 {
 	bust_spinlocks(0);
-	die_owner = -1;
 	add_taint(TAINT_DIE, LOCKDEP_NOW_UNRELIABLE);
 	die_nest_count--;
 	oops_exit();
 	printk("\n");
-	if (!die_nest_count)
+	if (!die_nest_count) {
 		/* Nest count reaches zero, release the lock. */
+		die_owner = -1;
 		arch_spin_unlock(&die_lock);
+	}
 	raw_local_irq_restore(flags);
 
 	crash_fadump(regs, "die oops");
-- 
2.10.2

Re: [2/2] powerpc: fix second nested oops hang

[Michael Ellerman]

On Tue, 2016-11-08 at 12:14:45 UTC, Nicholas Piggin wrote:
> When ending an oops, don't clear die_owner unless the nest count
> went to zero. This prevents a second nested oops from hanging forever
> on the die_lock.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/7458e8b2ce62e65051fec0bcb14cac

cheers