linux-kernel - [PATCH] powerpc/crash: Allow direct printing on kexec

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Message-Id: <20241020071755.328706-1-ryotkkr98@gmail.com>
Date: Sun, 20 Oct 2024 16:17:55 +0900
From: Ryo Takakura <ryotkkr98@...il.com>
To: mpe@...erman.id.au,
	npiggin@...il.com,
	christophe.leroy@...roup.eu,
	naveen@...nel.org,
	maddy@...ux.ibm.com,
	sourabhjain@...ux.ibm.com,
	hbathini@...ux.ibm.com,
	leobras.c@...il.com,
	pmladek@...e.com,
	john.ogness@...utronix.de
Cc: linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org,
	Ryo Takakura <ryotkkr98@...il.com>
Subject: [PATCH] powerpc/crash: Allow direct printing on kexec

Since the commit af2876b501e4 ("powerpc/crash: Use NMI context for printk
when starting to crash"), printing has been deferred before shutting down
non-panicked CPU on kexec to avoid deadlock on logbuf_lock. It is deferred
until the shutdown of the first kernel and starts booting into the second
kernel. As a result, there is no messages printed for legacy consoles,
including crash_kexec_post_notifiers messages which is after the
syncing of legacy console at printk_legacy_allow_panic_sync().

Let legacy consoles print directly so that we can see messages on kexec, as
the commit b6cf8b3f3312 ("printk: add lockless ringbuffer") turned printk
ring buffer lockless and there should be no worries panicked CPU 
deadlocking writing into ringbuffer after shutting down non-panicked CPU.

Signed-off-by: Ryo Takakura <ryotkkr98@...il.com>
---

Hi!

My understanding is that deferred printing can also be safely removed 
in terms of console lock as the commit d51507098ff9 ("printk: disable 
optimistic spin during panic") prevented from spinning in case of panic.

Sincerely,
Ryo Takakura

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

diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
index 9ac3266e4965..5e5260e0d964 100644
--- a/arch/powerpc/kexec/crash.c
+++ b/arch/powerpc/kexec/crash.c
@@ -269,9 +269,6 @@ static inline void crash_kexec_wait_realmode(int cpu) {}

 void crash_kexec_prepare(void)
 {
-	/* Avoid hardlocking with irresponsive CPU holding logbuf_lock */
-	printk_deferred_enter();
-
 	/*
 	 * This function is only called after the system
 	 * has panicked or is otherwise in a critical state.
-- 
2.34.1