ftrace: Prevent RCU stall on PREEMPT_VOLUNTARY kernels

The function match_records() may take a while due to a large
number of string comparisons, so when in PREEMPT_VOLUNTARY
kernels we could face RCU stalls due to that.

Add a cond_resched() to prevent that.

Link: https://lkml.kernel.org/r/[email protected]

Cc: Mark Rutland <[email protected]>
Suggested-by: Steven Rostedt <[email protected]>
Acked-by: Paul E. McKenney <[email protected]> # from RCU CPU stall warning perspective
Signed-off-by: Guilherme G. Piccoli <[email protected]>
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

kernel/trace/ftrace.c

@@ -4204,6 +4204,7 @@ match_records(struct ftrace_hash *hash, char *func, int len, char *mod)
 			}
 			found = 1;
 		}
+		cond_resched();
 	} while_for_each_ftrace_rec();
  out_unlock:
 	mutex_unlock(&ftrace_lock);