kcsan: Add test to generate conflicts via debugfs

Add 'test=<iters>' option to KCSAN's debugfs interface to invoke KCSAN
checks on a dummy variable. By writing 'test=<iters>' to the debugfs
file from multiple tasks, we can generate real conflicts, and trigger
data race reports.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/kcsan/debugfs.c

@@ -6,6 +6,7 @@
 #include <linux/debugfs.h>
 #include <linux/init.h>
 #include <linux/kallsyms.h>
+#include <linux/sched.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
 #include <linux/sort.h>
@@ -69,9 +70,9 @@ void kcsan_counter_dec(enum kcsan_counter_id id)
 /*
  * The microbenchmark allows benchmarking KCSAN core runtime only. To run
  * multiple threads, pipe 'microbench=<iters>' from multiple tasks into the
- * debugfs file.
+ * debugfs file. This will not generate any conflicts, and tests fast-path only.
  */
-static void microbenchmark(unsigned long iters)
+static noinline void microbenchmark(unsigned long iters)
 {
 	cycles_t cycles;
 
@@ -81,18 +82,52 @@ static void microbenchmark(unsigned long iters)
 	while (iters--) {
 		/*
 		 * We can run this benchmark from multiple tasks; this address
-		 * calculation increases likelyhood of some accesses overlapping
-		 * (they still won't conflict because all are reads).
+		 * calculation increases likelyhood of some accesses
+		 * overlapping. Make the access type an atomic read, to never
+		 * set up watchpoints and test the fast-path only.
 		 */
 		unsigned long addr =
 			iters % (CONFIG_KCSAN_NUM_WATCHPOINTS * PAGE_SIZE);
-		__kcsan_check_read((void *)addr, sizeof(long));
+		__kcsan_check_access((void *)addr, sizeof(long), KCSAN_ACCESS_ATOMIC);
 	}
 	cycles = get_cycles() - cycles;
 
 	pr_info("KCSAN: %s end   | cycles: %llu\n", __func__, cycles);
 }
 
+/*
+ * Simple test to create conflicting accesses. Write 'test=<iters>' to KCSAN's
+ * debugfs file from multiple tasks to generate real conflicts and show reports.
+ */
+static long test_dummy;
+static noinline void test_thread(unsigned long iters)
+{
+	const struct kcsan_ctx ctx_save = current->kcsan_ctx;
+	cycles_t cycles;
+
+	/* We may have been called from an atomic region; reset context. */
+	memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
+
+	pr_info("KCSAN: %s begin | iters: %lu\n", __func__, iters);
+
+	cycles = get_cycles();
+	while (iters--) {
+		__kcsan_check_read(&test_dummy, sizeof(test_dummy));
+		__kcsan_check_write(&test_dummy, sizeof(test_dummy));
+		ASSERT_EXCLUSIVE_WRITER(test_dummy);
+		ASSERT_EXCLUSIVE_ACCESS(test_dummy);
+
+		/* not actually instrumented */
+		WRITE_ONCE(test_dummy, iters);  /* to observe value-change */
+	}
+	cycles = get_cycles() - cycles;
+
+	pr_info("KCSAN: %s end   | cycles: %llu\n", __func__, cycles);
+
+	/* restore context */
+	current->kcsan_ctx = ctx_save;
+}
+
 static int cmp_filterlist_addrs(const void *rhs, const void *lhs)
 {
 	const unsigned long a = *(const unsigned long *)rhs;
@@ -242,6 +277,12 @@ debugfs_write(struct file *file, const char __user *buf, size_t count, loff_t *o
 		if (kstrtoul(&arg[sizeof("microbench=") - 1], 0, &iters))
 			return -EINVAL;
 		microbenchmark(iters);
+	} else if (!strncmp(arg, "test=", sizeof("test=") - 1)) {
+		unsigned long iters;
+
+		if (kstrtoul(&arg[sizeof("test=") - 1], 0, &iters))
+			return -EINVAL;
+		test_thread(iters);
 	} else if (!strcmp(arg, "whitelist")) {
 		set_report_filterlist_whitelist(true);
 	} else if (!strcmp(arg, "blacklist")) {