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Date: Thu, 13 Jun 2024 14:52:20 +0200
From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org
Subject: [PATCH] printk: nbcon_atomic_flush_pending() is safe only when there
 is no boot console
Message-ID: <ZmrrhCf5GRUdGwh9@pathway.suse.cz>
References: <20240603232453.33992-1-john.ogness@linutronix.de>
 <20240603232453.33992-6-john.ogness@linutronix.de>
 <ZmrpxWxUb8k4swrA@pathway.suse.cz>
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In-Reply-To: <ZmrpxWxUb8k4swrA@pathway.suse.cz>

Boot consoles are not serialized with the nbcon consoles via the nbcon
console context or con->device_lock(). The serialization is possible only
via the legacy console_lock().

The decision whether nbcon_atomic_flush_pending() should and can be
called safely is similar and closely related to the decision
whether the legacy loop should be used.

Define printing_via_context_safe symmetrically with printing_via_unlock.
Allow to call nbcon_atomic_flush_pending() only when it is needed and safe.

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/internal.h | 10 ++++++++++
 kernel/printk/nbcon.c    |  9 +++++++--
 kernel/printk/printk.c   |  7 ++++---
 3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 38680c6b2b39..bafec0a27da3 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -182,6 +182,7 @@ static inline bool console_is_usable(struct console *con, short flags) { return
 #endif /* CONFIG_PRINTK */
 
 extern bool have_boot_console;
+extern bool have_nbcon_console;
 extern bool have_legacy_console;
 
 /*
@@ -192,6 +193,15 @@ extern bool have_legacy_console;
  */
 #define printing_via_unlock (have_legacy_console || have_boot_console)
 
+/*
+ * Specifies if printing on nbcon consoles is needed and also safe
+ * when serialized only by the nbcon context. If @have_boot_console
+ * is true, the nbcon consoles must be serialized with the boot
+ * consoles using the legacy console_lock().
+ */
+#define printing_via_context_safe (have_nbcon_console || !have_boot_console)
+
+
 extern struct printk_buffers printk_shared_pbufs;
 
 /**
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 89b340ca303c..9b2df848718c 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1404,7 +1404,8 @@ void nbcon_cpu_emergency_exit(void)
 	 * for the emergency messages is NBCON_PRIO_EMERGENCY.
 	 */
 	if (*cpu_emergency_nesting == 1) {
-		nbcon_atomic_flush_pending();
+		if (printing_via_context_safe)
+			nbcon_atomic_flush_pending();
 
 		/*
 		 * Safely attempt to flush the legacy consoles in this
@@ -1446,7 +1447,8 @@ void nbcon_cpu_emergency_flush(void)
 	if (*(nbcon_get_cpu_emergency_nesting()) == 0)
 		return;
 
-	nbcon_atomic_flush_pending();
+	if (printing_via_context_safe)
+		nbcon_atomic_flush_pending();
 
 	if (printing_via_unlock && !is_printk_deferred()) {
 		if (console_trylock())
@@ -1637,6 +1639,9 @@ void nbcon_device_release(struct console *con)
 	 * was locked. The console_srcu_read_lock must be taken to ensure
 	 * the console is usable throughout flushing.
 	 */
+	if (!printing_via_context_safe)
+		return;
+
 	cookie = console_srcu_read_lock();
 	if (console_is_usable(con, console_srcu_read_flags(con)) &&
 	    !con->kthread &&
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 137bd9a721c4..3183db5b4180 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -475,7 +475,7 @@ bool have_legacy_console;
  * synchronous printing of legacy consoles will not occur during panic until
  * the backtrace has been stored to the ringbuffer.
  */
-static bool have_nbcon_console;
+bool have_nbcon_console;
 
 /*
  * Specifies if a boot console is registered. If boot consoles are present,
@@ -2370,7 +2370,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 
 	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
 
-	if (have_nbcon_console && !have_boot_console) {
+	if (printing_via_context_safe) {
 		bool is_panic_context = this_cpu_in_panic();
 
 		/*
@@ -3283,7 +3283,8 @@ void console_flush_on_panic(enum con_flush_mode mode)
 	if (mode == CONSOLE_REPLAY_ALL)
 		__console_rewind_all();
 
-	nbcon_atomic_flush_pending();
+	if (printing_via_context_safe)
+		nbcon_atomic_flush_pending();
 
 	if (printing_via_unlock)
 		console_flush_all(false, &next_seq, &handover);
-- 
2.45.2