fault-inject: use prandom where cryptographically secure randomness is not needed

Currently get_random*() is used to determine the probability of fault
injection, but cryptographically secure random numbers are not required.

There is no big problem in using prandom instead of get_random*() to
determine the probability of fault injection, and it also avoids acquiring
a spinlock, which is unsafe in some contexts.

[[e-mail]: tweak and reflow comment]
Link: https://lore.kernel.org/lkml/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Akinobu Mita <[e-mail]>
Suggested-by: Peter Zijlstra <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Masami Hiramatsu <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/fault-inject.c

···
 // SPDX-License-Identifier: GPL-2.0-only
 #include <linux/kernel.h>
 #include <linux/init.h>
-#include <linux/random.h>
+#include <linux/prandom.h>
 #include <linux/debugfs.h>
 #include <linux/sched.h>
 #include <linux/stat.h>
···
 #include <linux/interrupt.h>
 #include <linux/stacktrace.h>
 #include <linux/fault-inject.h>
+
+/*
+ * The should_fail() functions use prandom instead of the normal Linux RNG
+ * since they don't need cryptographically secure random numbers.
+ */
+static DEFINE_PER_CPU(struct rnd_state, fault_rnd_state);
+
+static u32 fault_prandom_u32_below_100(void)
+{
+	struct rnd_state *state;
+	u32 res;
+
+	state = &get_cpu_var(fault_rnd_state);
+	res = prandom_u32_state(state);
+	put_cpu_var(fault_rnd_state);
+
+	return res % 100;
+}
 
 /*
  * setup_fault_attr() is a helper function for various __setup handlers, so it
···
 			"FAULT_INJECTION: failed to parse arguments\n");
 		return 0;
 	}
+
+	prandom_init_once(&fault_rnd_state);
 
 	attr->probability = probability;
 	attr->interval = interval;
···
 			return false;
 	}
 
-	if (attr->probability <= get_random_u32_below(100))
+	if (attr->probability <= fault_prandom_u32_below_100())
 		return false;
 
 fail:
···
 	dir = debugfs_create_dir(name, parent);
 	if (IS_ERR(dir))
 		return dir;
+
+	prandom_init_once(&fault_rnd_state);
 
 	debugfs_create_ul("probability", mode, dir, &attr->probability);
 	debugfs_create_ul("interval", mode, dir, &attr->interval);
···
 
 void fault_config_init(struct fault_config *config, const char *name)
 {
+	prandom_init_once(&fault_rnd_state);
+
 	config_group_init_type_name(&config->group, name, &fault_config_type);
 }
 EXPORT_SYMBOL_GPL(fault_config_init);