[RFC][PATCH 4/6] serial: add uart port locking helpers

UART port spin_lock is one of the locks that can cause a
printk-recursion deadlock, thus we need to lock it from
printk_safe context. An example of a possible deadlock:

   IRQ
   foo_console_handle_IRQ()
    spin_lock(&uart_port->lock)
     tty_wakeup()
      printk()
       call_console_drivers()
        foo_console_write()
         spin_lock(&uart_port->lock)  << deadlock

This patch adds uart_port->lock helper macros which take care
of printk_safe context and redirect potentially unsafe printk()
calls to per-CPU buffers, which we flush later from safe a
context. The helper macros will turn the above mentioned deadlock
scenario into:

   IRQ
   foo_console_handle_IRQ()
    printk_safe_enter()
    spin_lock(&uart_port->lock)
     tty_wakeup()
      printk()
       printk_safe_log_store()
        irq_work_queue()
    spin_unlock(&uart_port->lock)
    printk_safe_exit()
   iret

   IRQ
    printk_safe_flush_buffer()
     vprintk_deferred()

Signed-off-by: Sergey Senozhatsky <redacted>
---
 include/linux/serial_core.h | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 06ea4eeb09ab..deb464520946 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h

@@ -256,6 +256,41 @@ struct uart_port {
 	void			*private_data;		/* generic platform data pointer */
 };
 
+#define uart_port_lock_irq(lock)				\
+	do {							\
+		printk_safe_enter_irq();			\
+		spin_lock(lock);				\
+	} while (0)
+
+#define uart_port_unlock_irq(lock)				\
+	do {							\
+		spin_unlock(lock);				\
+		printk_safe_exit_irq();				\
+	} while (0)
+
+#define uart_port_lock_irqsave(lock, flags)			\
+	do {							\
+		printk_safe_enter_irqsave(flags);		\
+		spin_lock(lock);				\
+	} while (0)
+
+#define uart_port_trylock_irqsave(lock, flags)			\
+	({							\
+		int locked;					\
+								\
+		printk_safe_enter_irqsave(flags);		\
+		locked = spin_trylock(lock);			\
+		if (!locked)					\
+			printk_safe_exit_irqrestore(flags);	\
+		locked;						\
+	})
+
+#define uart_port_unlock_irqrestore(lock, flags)		\
+	do {							\
+		spin_unlock(lock);				\
+		printk_safe_exit_irqrestore(flags);		\
+	} while (0)
+
 static inline int serial_port_in(struct uart_port *up, int offset)
 {
 	return up->serial_in(up, offset);

-- 
2.17.1