[PATCH v2 1/1] powerpc/crash: Use NMI context for printk when starting to crash

Currently, if printk lock (logbuf_lock) is held by other thread during
crash, there is a chance of deadlocking the crash on next printk, and
blocking a possibly desired kdump.

At the start of default_machine_crash_shutdown, make printk enter
NMI context, as it will use per-cpu buffers to store the message,
and avoid locking logbuf_lock.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Leonardo Bras <redacted>

---
Changes since v1:
- Added in-code comment explaining the need of context change
- Function moved to the start of default_machine_crash_shutdown,
  to avoid locking any printk on crashing routine.
- Title was 'Use NMI context for printk after crashing other CPUs'

---
 arch/powerpc/kexec/crash.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
index d488311efab1..c9a889880214 100644
--- a/arch/powerpc/kexec/crash.c
+++ b/arch/powerpc/kexec/crash.c

@@ -311,6 +311,9 @@ void default_machine_crash_shutdown(struct pt_regs *regs)
 	unsigned int i;
 	int (*old_handler)(struct pt_regs *regs);
 
+	/* Avoid hardlocking with irresponsive CPU holding logbuf_lock */
+	printk_nmi_enter();
+
 	/*
 	 * This function is only called after the system
 	 * has panicked or is otherwise in a critical state.

-- 
2.25.4

Excerpts from Leonardo Bras's message of May 13, 2020 7:45 am:

Currently, if printk lock (logbuf_lock) is held by other thread during
crash, there is a chance of deadlocking the crash on next printk, and
blocking a possibly desired kdump.

At the start of default_machine_crash_shutdown, make printk enter
NMI context, as it will use per-cpu buffers to store the message,
and avoid locking logbuf_lock.

printk_nmi_enter is used in one other place outside nmi_enter.

Is there a different/better way to handle this? What do other 
architectures do?

Other subsystems get put into an nmi-mode when we call nmi_enter
(lockdep, ftrace, rcu etc). It seems like those would be useful for 
similar reasons, so at least explaining why that is not used in a 
comment would be good.

Aside from that, I welcome any effort to make our crashes more reliable
so thanks for working on this stuff.

Thanks,
Nick

Hello Nick, thanks for your feedback.
Comments inline:

On Wed, 2020-05-13 at 14:36 +1000, Nicholas Piggin wrote:

Excerpts from Leonardo Bras's message of May 13, 2020 7:45 am:

quoted

Currently, if printk lock (logbuf_lock) is held by other thread during
crash, there is a chance of deadlocking the crash on next printk, and
blocking a possibly desired kdump.

At the start of default_machine_crash_shutdown, make printk enter
NMI context, as it will use per-cpu buffers to store the message,
and avoid locking logbuf_lock.

printk_nmi_enter is used in one other place outside nmi_enter.

Is there a different/better way to handle this? What do other 
architectures do?

To be honest, I was unaware of nmi_enter() and I have yet to study what
other architectures do here.

Other subsystems get put into an nmi-mode when we call nmi_enter
(lockdep, ftrace, rcu etc). It seems like those would be useful for 
similar reasons, so at least explaining why that is not used in a 
comment would be good.

My reasoning for using printk_nmi_enter() here was only to keep it from
using printk regular buffer (and locking logbuf_lock) at this point of
the crash.

I have yet to see how nmi_enter() extra functions would happen to
interfere with the crash at this point. 

(In a quick look at x86, (native_machine_crash_shutdown) I could not
see it using any printk, so it may not be necessary).

Aside from that, I welcome any effort to make our crashes more reliable
so thanks for working on this stuff.

Thanks,
Nick

Thank you, it means a lot.

Leonardo Bras

On Tue, 12 May 2020 18:45:35 -0300, Leonardo Bras wrote:

Currently, if printk lock (logbuf_lock) is held by other thread during
crash, there is a chance of deadlocking the crash on next printk, and
blocking a possibly desired kdump.

At the start of default_machine_crash_shutdown, make printk enter
NMI context, as it will use per-cpu buffers to store the message,
and avoid locking logbuf_lock.

Applied to powerpc/next.

[1/1] powerpc/crash: Use NMI context for printk when starting to crash
      https://git.kernel.org/powerpc/c/af2876b501e42c3fb5174cac9dd02598436f0fdf

cheers