printk() - do not merge continuation lines of different threads

This prevents the merging of printk() continuation lines of different
threads, in the case they race against each other.

It should properly isolate "atomic" single-line printk() users from
continuation users, to make sure the single-line users will never be
merged with the racy continuation ones.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Kay Sievers <kay@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/printk.c

@@ -1230,12 +1230,13 @@ asmlinkage int vprintk_emit(int facility, int level,
 	static size_t buflen;
 	static int buflevel;
 	static char textbuf[LOG_LINE_MAX];
+	static struct task_struct *cont;
 	char *text = textbuf;
 	size_t textlen;
 	unsigned long flags;
 	int this_cpu;
 	bool newline = false;
-	bool cont = false;
+	bool prefix = false;
 	int printed_len = 0;
 
 	boot_delay_msec();
@@ -1295,20 +1296,16 @@ asmlinkage int vprintk_emit(int facility, int level,
 		case '0' ... '7':
 			if (level == -1)
 				level = text[1] - '0';
-			text += 3;
-			textlen -= 3;
-			break;
-		case 'c':	/* KERN_CONT */
-			cont = true;
 		case 'd':	/* KERN_DEFAULT */
+			prefix = true;
+		case 'c':	/* KERN_CONT */
 			text += 3;
 			textlen -= 3;
-			break;
 		}
 	}
 
-	if (buflen && (!cont || dict)) {
-		/* no continuation; flush existing buffer */
+	if (buflen && (prefix || dict || cont != current)) {
+		/* flush existing buffer */
 		log_store(facility, buflevel, NULL, 0, buf, buflen);
 		printed_len += buflen;
 		buflen = 0;
@@ -1342,6 +1339,10 @@ asmlinkage int vprintk_emit(int facility, int level,
 				  dict, dictlen, text, textlen);
 			printed_len += textlen;
 		}
+		cont = NULL;
+	} else {
+		/* remember thread which filled the buffer */
+		cont = current;
 	}
 
 	/*