printk: queue wake_up_klogd irq_work only if per-CPU areas are ready

printk_deferred(), similarly to printk_safe/printk_nmi, does not
immediately attempt to print a new message on the consoles, avoiding
calls into non-reentrant kernel paths, e.g. scheduler or timekeeping,
which potentially can deadlock the system.

Those printk() flavors, instead, rely on per-CPU flush irq_work to print
messages from safer contexts. For same reasons (recursive scheduler or
timekeeping calls) printk() uses per-CPU irq_work in order to wake up
user space syslog/kmsg readers.

However, only printk_safe/printk_nmi do make sure that per-CPU areas
have been initialised and that it's safe to modify per-CPU irq_work.
This means that, for instance, should printk_deferred() be invoked "too
early", that is before per-CPU areas are initialised, printk_deferred()
will perform illegal per-CPU access.

Lech Perczak [0] reports that after commit 1b710b1b10ef ("char/random:
silence a lockdep splat with printk()") user-space syslog/kmsg readers
are not able to read new kernel messages.

The reason is printk_deferred() being called too early (as was pointed
out by Petr and John).

Fix printk_deferred() and do not queue per-CPU irq_work before per-CPU
areas are initialized.

Link: https://lore.kernel.org/lkml/[e-mail]/
Reported-by: Lech Perczak <[e-mail]>
Signed-off-by: Sergey Senozhatsky <[e-mail]>
Tested-by: Jann Horn <[e-mail]>
Reviewed-by: Petr Mladek <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Theodore Ts'o <[e-mail]>
Cc: John Ogness <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/printk.h

···
 void dump_stack_print_info(const char *log_lvl);
 void show_regs_print_info(const char *log_lvl);
 extern asmlinkage void dump_stack(void) __cold;
-extern void printk_safe_init(void);
 extern void printk_safe_flush(void);
 extern void printk_safe_flush_on_panic(void);
 #else
···
 }
 
 static inline void dump_stack(void)
-{
-}
-
-static inline void printk_safe_init(void)
 {
 }
 

init/main.c

···
 	boot_init_stack_canary();
 
 	time_init();
-	printk_safe_init();
 	perf_event_init();
 	profile_init();
 	call_function_init();

kernel/printk/internal.h

···
 void __printk_safe_enter(void);
 void __printk_safe_exit(void);
 
+void printk_safe_init(void);
+bool printk_percpu_data_ready(void);
+
 #define printk_safe_enter_irqsave(flags)	\
 	do {					\
 		local_irq_save(flags);		\
···
 #define printk_safe_enter_irq() local_irq_disable()
 #define printk_safe_exit_irq() local_irq_enable()
 
+static inline void printk_safe_init(void) { }
+static inline bool printk_percpu_data_ready(void) { return false; }
 #endif /* CONFIG_PRINTK */

kernel/printk/printk.c

···
 static char *log_buf = __log_buf;
 static u32 log_buf_len = __LOG_BUF_LEN;
 
+/*
+ * We cannot access per-CPU data (e.g. per-CPU flush irq_work) before
+ * per_cpu_areas are initialised. This variable is set to true when
+ * it's safe to access per-CPU data.
+ */
+static bool __printk_percpu_data_ready __read_mostly;
+
+bool printk_percpu_data_ready(void)
+{
+	return __printk_percpu_data_ready;
+}
+
 /* Return log buffer address */
 char *log_buf_addr_get(void)
 {
···
 static inline void log_buf_add_cpu(void) {}
 #endif /* CONFIG_SMP */
 
+static void __init set_percpu_data_ready(void)
+{
+	printk_safe_init();
+	/* Make sure we set this flag only after printk_safe() init is done */
+	barrier();
+	__printk_percpu_data_ready = true;
+}
+
 void __init setup_log_buf(int early)
 {
 	unsigned long flags;
 	char *new_log_buf;
 	unsigned int free;
+
+	/*
+	 * Some archs call setup_log_buf() multiple times - first is very
+	 * early, e.g. from setup_arch(), and second - when percpu_areas
+	 * are initialised.
+	 */
+	if (!early)
+		set_percpu_data_ready();
 
 	if (log_buf != __log_buf)
 		return;
···
 
 void wake_up_klogd(void)
 {
+	if (!printk_percpu_data_ready())
+		return;
+
 	preempt_disable();
 	if (waitqueue_active(&log_wait)) {
 		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
···
 
 void defer_console_output(void)
 {
+	if (!printk_percpu_data_ready())
+		return;
+
 	preempt_disable();
 	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
 	irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));

kernel/printk/printk_safe.c

···
  * There are situations when we want to make sure that all buffers
  * were handled or when IRQs are blocked.
  */
-static int printk_safe_irq_ready __read_mostly;
 
 #define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) -	\
 				sizeof(atomic_t) -			\
···
 /* Get flushed in a more safe context. */
 static void queue_flush_work(struct printk_safe_seq_buf *s)
 {
-	if (printk_safe_irq_ready)
+	if (printk_percpu_data_ready())
 		irq_work_queue(&s->work);
 }
 
···
 		init_irq_work(&s->work, __printk_safe_flush);
 #endif
 	}
-
-	/*
-	 * In the highly unlikely event that a NMI were to trigger at
-	 * this moment. Make sure IRQ work is set up before this
-	 * variable is set.
-	 */
-	barrier();
-	printk_safe_irq_ready = 1;
 
 	/* Flush pending messages that did not have scheduled IRQ works. */
 	printk_safe_flush();