revocable: Add KUnit test for concurrent access

Add a test case to verify correct synchronization between concurrent
readers and a revocation.

The test setup involves:
1. Consumer 1 enters the critical section (SRCU read lock) and verifies
   access to the resource.
2. Provider attempts to revoke the resource.  This should block until
   Consumer 1 releases the lock.
3. Consumer 2 attempts to enter the critical section while revocation
   is pending.  It should see the resource as revoked (NULL).
4. Consumer 1 exits, allowing the revocation to complete.

This ensures that the SRCU mechanism correctly enforces grace periods
and that new readers are properly prevented from accessing the resource
once revocation has begun.

A way to run the test:
$ ./tools/testing/kunit/kunit.py run \
	--kconfig_add CONFIG_REVOCABLE_KUNIT_TEST=y \
	--kconfig_add CONFIG_PROVE_LOCKING=y \
	--kconfig_add CONFIG_DEBUG_KERNEL=y \
	--kconfig_add CONFIG_DEBUG_INFO=y \
	--kconfig_add CONFIG_DEBUG_INFO_DWARF5=y \
	--kconfig_add CONFIG_KASAN=y \
	--kconfig_add CONFIG_DETECT_HUNG_TASK=y \
	--kconfig_add CONFIG_DEFAULT_HUNG_TASK_TIMEOUT="10" \
	--arch=x86_64 --raw_output=all \
	revocable_test

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://patch.msgid.link/20260129143733.45618-5-tzungbi@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Tzung-Bi Shih

drivers/base/revocable_test.c

@@ -17,9 +17,14 @@
  *
  * - Provider Use-after-free: Verifies revocable_init() correctly handles
  *   race conditions where the provider is being released.
+ *
+ * - Concurrent Access: Verifies multiple threads can access the resource.
  */
 
 #include <kunit/test.h>
+#include <linux/completion.h>
+#include <linux/delay.h>
+#include <linux/kthread.h>
 #include <linux/refcount.h>
 #include <linux/revocable.h>
 
@@ -160,12 +165,111 @@ static void revocable_test_provider_use_after_free(struct kunit *test)
 	KUNIT_EXPECT_NE(test, ret, 0);
 }
 
+struct test_concurrent_access_context {
+	struct kunit *test;
+	struct revocable_provider __rcu *rp;
+	struct revocable rev;
+	struct completion started, enter, exit;
+	struct task_struct *thread;
+	void *expected_res;
+};
+
+static int test_concurrent_access_provider(void *data)
+{
+	struct test_concurrent_access_context *ctx = data;
+
+	complete(&ctx->started);
+
+	wait_for_completion(&ctx->enter);
+	revocable_provider_revoke(&ctx->rp);
+	KUNIT_EXPECT_PTR_EQ(ctx->test, unrcu_pointer(ctx->rp), NULL);
+
+	return 0;
+}
+
+static int test_concurrent_access_consumer(void *data)
+{
+	struct test_concurrent_access_context *ctx = data;
+	void *res;
+
+	complete(&ctx->started);
+
+	wait_for_completion(&ctx->enter);
+	res = revocable_try_access(&ctx->rev);
+	KUNIT_EXPECT_PTR_EQ(ctx->test, res, ctx->expected_res);
+
+	wait_for_completion(&ctx->exit);
+	revocable_withdraw_access(&ctx->rev);
+
+	return 0;
+}
+
+static void revocable_test_concurrent_access(struct kunit *test)
+{
+	struct revocable_provider __rcu *rp;
+	void *real_res = (void *)0x12345678;
+	struct test_concurrent_access_context *ctx;
+	int ret, i;
+
+	rp = revocable_provider_alloc(real_res);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, rp);
+
+	ctx = kunit_kmalloc_array(test, 3, sizeof(*ctx), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, ctx);
+
+	for (i = 0; i < 3; ++i) {
+		ctx[i].test = test;
+		init_completion(&ctx[i].started);
+		init_completion(&ctx[i].enter);
+		init_completion(&ctx[i].exit);
+
+		if (i == 0) {
+			ctx[i].rp = rp;
+			ctx[i].thread = kthread_run(
+				test_concurrent_access_provider, ctx + i,
+				"revocable_provider_%d", i);
+		} else {
+			ret = revocable_init(rp, &ctx[i].rev);
+			KUNIT_ASSERT_EQ(test, ret, 0);
+
+			ctx[i].thread = kthread_run(
+				test_concurrent_access_consumer, ctx + i,
+				"revocable_consumer_%d", i);
+		}
+		KUNIT_ASSERT_FALSE(test, IS_ERR(ctx[i].thread));
+
+		wait_for_completion(&ctx[i].started);
+	}
+	ctx[1].expected_res = real_res;
+	ctx[2].expected_res = NULL;
+
+	/* consumer1 enters read-side critical section */
+	complete(&ctx[1].enter);
+	msleep(100);
+	/* provider0 revokes the resource */
+	complete(&ctx[0].enter);
+	msleep(100);
+	/* consumer2 enters read-side critical section */
+	complete(&ctx[2].enter);
+	msleep(100);
+
+	/* consumer{1,2} exit read-side critical section */
+	complete(&ctx[1].exit);
+	complete(&ctx[2].exit);
+
+	for (i = 0; i < 3; ++i)
+		kthread_stop(ctx[i].thread);
+	for (i = 1; i < 3; ++i)
+		revocable_deinit(&ctx[i].rev);
+}
+
 static struct kunit_case revocable_test_cases[] = {
 	KUNIT_CASE(revocable_test_basic),
 	KUNIT_CASE(revocable_test_revocation),
 	KUNIT_CASE(revocable_test_try_access_macro),
 	KUNIT_CASE(revocable_test_try_access_macro2),
 	KUNIT_CASE(revocable_test_provider_use_after_free),
+	KUNIT_CASE(revocable_test_concurrent_access),
 	{}
 };