printk: Defer legacy printing when holding printk_cpu_sync

The documentation of printk_cpu_sync_get() clearly states
that the owner must never perform any activities where it waits
for a CPU. For legacy printing there can be spinning on the
console_lock and on the port lock. Therefore legacy printing
must be deferred when holding the printk_cpu_sync.

Note that in the case of emergency states, atomic consoles
are not prevented from printing when printk is deferred. This
is appropriate because they do not spin-wait indefinitely for
other CPUs.

Reported-by: Rik van Riel <riel@surriel.com>
Closes: https://lore.kernel.org/r/20240715232052.73eb7fb1@imladris.surriel.com
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Fixes: 55d6af1d66 ("lib/nmi_backtrace: explicitly serialize banner and regs")
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20241209111746.192559-3-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

kernel/printk/internal.h

@@ -338,3 +338,9 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped);
 void console_prepend_replay(struct printk_message *pmsg);
 #endif
+
+#ifdef CONFIG_SMP
+bool is_printk_cpu_sync_owner(void);
+#else
+static inline bool is_printk_cpu_sync_owner(void) { return false; }
+#endif

kernel/printk/printk.c

@@ -4922,6 +4922,11 @@ void console_try_replay_all(void)
 static atomic_t printk_cpu_sync_owner = ATOMIC_INIT(-1);
 static atomic_t printk_cpu_sync_nested = ATOMIC_INIT(0);
 
+bool is_printk_cpu_sync_owner(void)
+{
+	return (atomic_read(&printk_cpu_sync_owner) == raw_smp_processor_id());
+}
+
 /**
  * __printk_cpu_sync_wait() - Busy wait until the printk cpu-reentrant
  *                            spinning lock is not owned by any CPU.

kernel/printk/printk_safe.c

@@ -61,10 +61,15 @@ bool is_printk_legacy_deferred(void)
 	/*
 	 * The per-CPU variable @printk_context can be read safely in any
 	 * context. CPU migration is always disabled when set.
+	 *
+	 * A context holding the printk_cpu_sync must not spin waiting for
+	 * another CPU. For legacy printing, it could be the console_lock
+	 * or the port lock.
 	 */
 	return (force_legacy_kthread() ||
 		this_cpu_read(printk_context) ||
-		in_nmi());
+		in_nmi() ||
+		is_printk_cpu_sync_owner());
 }
 
 asmlinkage int vprintk(const char *fmt, va_list args)