linux-kernel - [PATCH v3 1/1] powerpc/crash: Use NMI context for printk when starting to crash

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Date:   Mon, 18 May 2020 21:25:58 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Allison Randal <allison@...utok.net>,
        Leonardo Bras <leobras.c@...il.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Alexios Zavras <alexios.zavras@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: [PATCH v3 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@...erman.id.au>
Signed-off-by: Leonardo Bras <leobras.c@...il.com>

---
Changes since v2:
- Changes usage of printk_nmi_enter() to nmi_enter()
  (Suggested by Nick Piggin)

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 | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
index d488311efab1..53c5cf9b6d3c 100644
--- a/arch/powerpc/kexec/crash.c
+++ b/arch/powerpc/kexec/crash.c
@@ -16,6 +16,7 @@
 #include <linux/delay.h>
 #include <linux/irq.h>
 #include <linux/types.h>
+#include <linux/hardirq.h>
 
 #include <asm/processor.h>
 #include <asm/machdep.h>
@@ -311,6 +312,13 @@ 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,
+	 * by using printk nmi_context
+	 */
+	if (!in_nmi())
+		nmi_enter();
+
 	/*
 	 * This function is only called after the system
 	 * has panicked or is otherwise in a critical state.
-- 
2.25.4