Fix locking bug in "acquire_console_semaphore_for_printk()"

When I cleaned up printk() and split up the printk locking logic in
commit 266c2e0abeca649fa6667a1a427ad1da507c6375 ("Make printk() console
semaphore accesses sensible") I had incorrectly moved the call to
have_callable_console() outside of the console semaphore.

That was buggy. The console semaphore protects the console_drivers list
that is used by have_callable_console().

Thanks go to Bongani Hlope who saw this as a hang on shutdown and reboot
and bisected the bug to the right commit, and tested this patch. See

http://lkml.org/lkml/2008/4/11/315

Bisected-and-tested-by: Bongani Hlope <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/printk.c

···
 {
 	int retval = 0;
 
-	if (can_use_console(cpu))
-		retval = !try_acquire_console_sem();
+	if (!try_acquire_console_sem()) {
+		retval = 1;
+
+		/*
+		 * If we can't use the console, we need to release
+		 * the console semaphore by hand to avoid flushing
+		 * the buffer. We need to hold the console semaphore
+		 * in order to do this test safely.
+		 */
+		if (!can_use_console(cpu)) {
+			console_locked = 0;
+			up(&console_sem);
+			retval = 0;
+		}
+	}
 	printk_cpu = UINT_MAX;
 	spin_unlock(&logbuf_lock);
 	return retval;