ratelimit: Count misses due to lock contention

The ___ratelimit() function simply returns zero ("do ratelimiting")
if the trylock fails, but does not adjust the ->missed field.  This
means that the resulting dropped printk()s are dropped silently, which
could seriously confuse people trying to do console-log-based debugging.
Therefore, increment the ->missed field upon trylock failure.

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

Author: paulmckrcu

lib/ratelimit.c

@@ -44,8 +44,10 @@ int ___ratelimit(struct ratelimit_state *rs, const char *func)
 	 * in addition to the one that will be printed by
 	 * the entity that is holding the lock already:
 	 */
-	if (!raw_spin_trylock_irqsave(&rs->lock, flags))
+	if (!raw_spin_trylock_irqsave(&rs->lock, flags)) {
+		ratelimit_state_inc_miss(rs);
 		return 0;
+	}
 
 	if (!rs->begin)
 		rs->begin = jiffies;