printk: Prepare for nested printk_nmi_enter()

There is plenty of space in the printk_context variable. Reserve one byte
there for the NMI context to be on the safe side.

It should never overflow. The BUG_ON(in_nmi() == NMI_MASK) in nmi_enter()
will trigger much earlier.
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Link: https://lkml.kernel.org/r/20200505134100.681374113@linutronix.de

kernel/printk/internal.h

@@ -6,9 +6,11 @@
 
 #ifdef CONFIG_PRINTK
 
-#define PRINTK_SAFE_CONTEXT_MASK	 0x3fffffff
-#define PRINTK_NMI_DIRECT_CONTEXT_MASK	 0x40000000
-#define PRINTK_NMI_CONTEXT_MASK		 0x80000000
+#define PRINTK_SAFE_CONTEXT_MASK	0x007ffffff
+#define PRINTK_NMI_DIRECT_CONTEXT_MASK	0x008000000
+#define PRINTK_NMI_CONTEXT_MASK		0xff0000000
+
+#define PRINTK_NMI_CONTEXT_OFFSET	0x010000000
 
 extern raw_spinlock_t logbuf_lock;
 

kernel/printk/printk_safe.c

@@ -295,12 +295,12 @@ static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
 
 void notrace printk_nmi_enter(void)
 {
-	this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
+	this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
 }
 
 void notrace printk_nmi_exit(void)
 {
-	this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
+	this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
 }
 
 /*