From 99f90e55308a835589018a30a37d29e60a64a1f7 Mon Sep 17 00:00:00 2001
From: John Ogness <john.ogness@linutronix.de>
Date: Mon, 30 Nov 2020 01:42:01 +0106
Subject: [PATCH 099/327] console: add write_atomic interface

Add a write_atomic() callback to the console. This is an optional
function for console drivers. The function must be atomic (including
NMI safe) for writing to the console.

Console drivers must still implement the write() callback. The
write_atomic() callback will only be used in special situations,
such as when the kernel panics.

Creating an NMI safe write_atomic() that must synchronize with
write() requires a careful implementation of the console driver. To
aid with the implementation, a set of console_atomic_*() functions
are provided:

    void console_atomic_lock(unsigned int *flags);
    void console_atomic_unlock(unsigned int flags);

These functions synchronize using a processor-reentrant spinlock
(called a cpulock).

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/console.h |   4 ++
 kernel/printk/printk.c  | 100 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 104 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index bc2a749e6f0d..613df76903f5 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -141,6 +141,7 @@ static inline int con_debug_leave(void)
 struct console {
 	char	name[16];
 	void	(*write)(struct console *, const char *, unsigned);
+	void	(*write_atomic)(struct console *co, const char *s, unsigned int count);
 	int	(*read)(struct console *, char *, unsigned);
 	struct tty_driver *(*device)(struct console *, int *);
 	void	(*unblank)(void);
@@ -232,4 +233,7 @@ extern void console_init(void);
 void dummycon_register_output_notifier(struct notifier_block *nb);
 void dummycon_unregister_output_notifier(struct notifier_block *nb);
 
+extern void console_atomic_lock(unsigned int *flags);
+extern void console_atomic_unlock(unsigned int flags);
+
 #endif /* _LINUX_CONSOLE_H */
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e7257fb4f446..3b41f2988579 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3580,3 +3580,103 @@ void kmsg_dump_rewind(struct kmsg_dumper_iter *iter)
 EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
 
 #endif
+
+struct prb_cpulock {
+	atomic_t owner;
+	unsigned long __percpu *irqflags;
+};
+
+#define DECLARE_STATIC_PRINTKRB_CPULOCK(name)				\
+static DEFINE_PER_CPU(unsigned long, _##name##_percpu_irqflags);	\
+static struct prb_cpulock name = {					\
+	.owner = ATOMIC_INIT(-1),					\
+	.irqflags = &_##name##_percpu_irqflags,				\
+}
+
+static bool __prb_trylock(struct prb_cpulock *cpu_lock,
+			  unsigned int *cpu_store)
+{
+	unsigned long *flags;
+	unsigned int cpu;
+
+	cpu = get_cpu();
+
+	*cpu_store = atomic_read(&cpu_lock->owner);
+	/* memory barrier to ensure the current lock owner is visible */
+	smp_rmb();
+	if (*cpu_store == -1) {
+		flags = per_cpu_ptr(cpu_lock->irqflags, cpu);
+		local_irq_save(*flags);
+		if (atomic_try_cmpxchg_acquire(&cpu_lock->owner,
+					       cpu_store, cpu)) {
+			return true;
+		}
+		local_irq_restore(*flags);
+	} else if (*cpu_store == cpu) {
+		return true;
+	}
+
+	put_cpu();
+	return false;
+}
+
+/*
+ * prb_lock: Perform a processor-reentrant spin lock.
+ * @cpu_lock: A pointer to the lock object.
+ * @cpu_store: A "flags" pointer to store lock status information.
+ *
+ * If no processor has the lock, the calling processor takes the lock and
+ * becomes the owner. If the calling processor is already the owner of the
+ * lock, this function succeeds immediately. If lock is locked by another
+ * processor, this function spins until the calling processor becomes the
+ * owner.
+ *
+ * It is safe to call this function from any context and state.
+ */
+static void prb_lock(struct prb_cpulock *cpu_lock, unsigned int *cpu_store)
+{
+	for (;;) {
+		if (__prb_trylock(cpu_lock, cpu_store))
+			break;
+		cpu_relax();
+	}
+}
+
+/*
+ * prb_unlock: Perform a processor-reentrant spin unlock.
+ * @cpu_lock: A pointer to the lock object.
+ * @cpu_store: A "flags" object storing lock status information.
+ *
+ * Release the lock. The calling processor must be the owner of the lock.
+ *
+ * It is safe to call this function from any context and state.
+ */
+static void prb_unlock(struct prb_cpulock *cpu_lock, unsigned int cpu_store)
+{
+	unsigned long *flags;
+	unsigned int cpu;
+
+	cpu = atomic_read(&cpu_lock->owner);
+	atomic_set_release(&cpu_lock->owner, cpu_store);
+
+	if (cpu_store == -1) {
+		flags = per_cpu_ptr(cpu_lock->irqflags, cpu);
+		local_irq_restore(*flags);
+	}
+
+	put_cpu();
+}
+
+DECLARE_STATIC_PRINTKRB_CPULOCK(printk_cpulock);
+
+void console_atomic_lock(unsigned int *flags)
+{
+	prb_lock(&printk_cpulock, flags);
+}
+EXPORT_SYMBOL(console_atomic_lock);
+
+void console_atomic_unlock(unsigned int flags)
+{
+	prb_unlock(&printk_cpulock, flags);
+}
+EXPORT_SYMBOL(console_atomic_unlock);
-- 
2.49.0

