ratelimit: Reduce ___ratelimit() false-positive rate limiting

Retain the locked design, but check rate-limiting even when the lock
could not be acquired.

Link: https://lore.kernel.org/all/[e-mail]/
Link: https://lore.kernel.org/all/fbe93a52-365e-47fe-93a4-44a44547d601@paulmck-laptop/
Link: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: Petr Mladek <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Cc: Petr Mladek <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Kuniyuki Iwashima <[e-mail]>
Cc: Mateusz Guzik <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: John Ogness <[e-mail]>
Cc: Sergey Senozhatsky <[e-mail]>

include/linux/ratelimit.h

···
 	raw_spin_lock_irqsave(&rs->lock, flags);
 	rs->interval = interval_init;
 	rs->flags &= ~RATELIMIT_INITIALIZED;
-	rs->printed = 0;
+	atomic_set(&rs->rs_n_left, rs->burst);
 	ratelimit_state_reset_miss(rs);
 	raw_spin_unlock_irqrestore(&rs->lock, flags);
 }

include/linux/ratelimit_types.h

···
 
 	int		interval;
 	int		burst;
-	int		printed;
+	atomic_t	rs_n_left;
 	atomic_t	missed;
 	unsigned int	flags;
 	unsigned long	begin;

lib/ratelimit.c

···
 		return 1;
 
 	/*
-	 * If we contend on this state's lock then almost
-	 * by definition we are too busy to print a message,
-	 * in addition to the one that will be printed by
-	 * the entity that is holding the lock already:
+	 * If we contend on this state's lock then just check if
+	 * the current burst is used or not. It might cause
+	 * false positive when we are past the interval and
+	 * the current lock owner is just about to reset it.
 	 */
 	if (!raw_spin_trylock_irqsave(&rs->lock, flags)) {
+		unsigned int rs_flags = READ_ONCE(rs->flags);
+
+		if (rs_flags & RATELIMIT_INITIALIZED && burst) {
+			int n_left;
+
+			n_left = atomic_dec_return(&rs->rs_n_left);
+			if (n_left >= 0)
+				return 1;
+		}
+
 		ratelimit_state_inc_miss(rs);
 		return 0;
 	}
···
 	if (!(rs->flags & RATELIMIT_INITIALIZED)) {
 		rs->begin = jiffies;
 		rs->flags |= RATELIMIT_INITIALIZED;
+		atomic_set(&rs->rs_n_left, rs->burst);
 	}
 
 	if (time_is_before_jiffies(rs->begin + interval)) {
-		int m = ratelimit_state_reset_miss(rs);
+		int m;
 
+		/*
+		 * Reset rs_n_left ASAP to reduce false positives
+		 * in parallel calls, see above.
+		 */
+		atomic_set(&rs->rs_n_left, rs->burst);
+		rs->begin = jiffies;
+
+		m = ratelimit_state_reset_miss(rs);
 		if (m) {
 			if (!(rs->flags & RATELIMIT_MSG_ON_RELEASE)) {
 				printk_deferred(KERN_WARNING
 						"%s: %d callbacks suppressed\n", func, m);
 			}
 		}
-		rs->begin   = jiffies;
-		rs->printed = 0;
 	}
-	if (burst && burst > rs->printed) {
-		rs->printed++;
-		ret = 1;
-	} else {
-		ratelimit_state_inc_miss(rs);
-		ret = 0;
+	if (burst) {
+		int n_left;
+
+		/* The burst might have been taken by a parallel call. */
+		n_left = atomic_dec_return(&rs->rs_n_left);
+		if (n_left >= 0) {
+			ret = 1;
+			goto unlock_ret;
+		}
 	}
+
+	ratelimit_state_inc_miss(rs);
+	ret = 0;
+
+unlock_ret:
 	raw_spin_unlock_irqrestore(&rs->lock, flags);
 
 	return ret;