linux-kernel - [PATCH printk v2 18/26] printk: nbcon: Add unsafe flushing on panic

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Message-Id: <20240218185726.1994771-19-john.ogness@linutronix.de>
Date: Sun, 18 Feb 2024 20:03:18 +0106
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Uros Bizjak <ubizjak@...il.com>,
	"Guilherme G. Piccoli" <gpiccoli@...lia.com>,
	Lukas Wunner <lukas@...ner.de>,
	Arnd Bergmann <arnd@...db.de>
Subject: [PATCH printk v2 18/26] printk: nbcon: Add unsafe flushing on panic

Add nbcon_atomic_flush_unsafe() to flush all nbcon consoles
using the write_atomic() callback and allowing unsafe hostile
takeovers. Call this at the end of panic() as a final attempt
to flush any pending messages.

Note that legacy consoles use unsafe methods for flushing
from the beginning of panic (see bust_spinlocks()). Therefore,
systems using both legacy and nbcon consoles may still fail to
see panic messages due to unsafe legacy console usage.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 include/linux/printk.h |  5 +++++
 kernel/panic.c         |  1 +
 kernel/printk/nbcon.c  | 18 ++++++++++++++++--
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index ef57a4d93ae2..fe37395af9e2 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -199,6 +199,7 @@ extern asmlinkage void dump_stack(void) __cold;
 void printk_trigger_flush(void);
 extern void uart_nbcon_acquire(struct uart_port *up);
 extern void uart_nbcon_release(struct uart_port *up);
+void nbcon_atomic_flush_unsafe(void);
 #else
 static inline __printf(1, 0)
 int vprintk(const char *s, va_list args)
@@ -287,6 +288,10 @@ static inline void uart_nbcon_release(struct uart_port *up)
 {
 }
 
+static inline void nbcon_atomic_flush_unsafe(void)
+{
+}
+
 #endif
 
 bool this_cpu_in_panic(void);
diff --git a/kernel/panic.c b/kernel/panic.c
index f22d8f33ea14..c039f8e1ddae 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -453,6 +453,7 @@ void panic(const char *fmt, ...)
 	 * Explicitly flush the kernel log buffer one last time.
 	 */
 	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
+	nbcon_atomic_flush_unsafe();
 
 	local_irq_enable();
 	for (i = 0; ; i += PANIC_TIMER_STEP) {
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 1a18549e43b8..64dedd79e880 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1025,8 +1025,9 @@ bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
  * __nbcon_atomic_flush_all - Flush all nbcon consoles using their
  *					write_atomic() callback
  * @stop_seq:			Flush up until this record
+ * @allow_unsafe_takeover:	True, to allow unsafe hostile takeovers
  */
-static void __nbcon_atomic_flush_all(u64 stop_seq)
+static void __nbcon_atomic_flush_all(u64 stop_seq, bool allow_unsafe_takeover)
 {
 	struct nbcon_write_context wctxt = { };
 	struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
@@ -1054,6 +1055,7 @@ static void __nbcon_atomic_flush_all(u64 stop_seq)
 			memset(ctxt, 0, sizeof(*ctxt));
 			ctxt->console			= con;
 			ctxt->spinwait_max_us		= 2000;
+			ctxt->allow_unsafe_takeover	= allow_unsafe_takeover;
 
 			/*
 			 * Atomic flushing does not use console driver
@@ -1088,7 +1090,19 @@ static void __nbcon_atomic_flush_all(u64 stop_seq)
  */
 void nbcon_atomic_flush_all(void)
 {
-	__nbcon_atomic_flush_all(prb_next_reserve_seq(prb));
+	__nbcon_atomic_flush_all(prb_next_reserve_seq(prb), false);
+}
+
+/**
+ * nbcon_atomic_flush_unsafe - Flush all nbcon consoles using their
+ *	write_atomic() callback and allowing unsafe hostile takeovers
+ *
+ * Flush the backlog up through the currently newest record. Unsafe hostile
+ * takeovers will be performed, if necessary.
+ */
+void nbcon_atomic_flush_unsafe(void)
+{
+	__nbcon_atomic_flush_all(prb_next_reserve_seq(prb), true);
 }
 
 /**
-- 
2.39.2