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

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Date:   Tue, 12 May 2020 18:45:35 -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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexios Zavras <alexios.zavras@...el.com>,
        Enrico Weigelt <info@...ux.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Leonardo Bras <leobras.c@...il.com>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: [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@...erman.id.au>
Signed-off-by: Leonardo Bras <leobras.c@...il.com>

---
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