Date:   Mon, 5 Dec 2022 09:57:12 +0530
From:   Sadiya Kazi <sadiyakazi@...gle.com>
To:     David Gow <davidgow@...gle.com>
Cc:     Brendan Higgins <brendan.higgins@...ux.dev>,
        Daniel Latypov <dlatypov@...gle.com>,
        Shuah Khan <skhan@...uxfoundation.org>,
        kunit-dev@...glegroups.com, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v4 2/3] kunit: Use the static key when retrieving the
 current test

On Fri, Nov 25, 2022 at 2:13 PM 'David Gow' via KUnit Development
<kunit-dev@...glegroups.com> wrote:
>
> In order to detect if a KUnit test is running, and to access its
> context, the 'kunit_test' member of the current task_struct is used.
> Usually, this is accessed directly or via the kunit_fail_current_task()
> function.
>
> In order to speed up the case where no test is running, add a wrapper,
> kunit_get_current_test(), which uses the static key to fail early.
> Equally, Speed up kunit_fail_current_test() by using the static key.
>
> This should make it convenient for code to call this
> unconditionally in fakes or error paths, without worrying that this will
> slow the code down significantly.
>
> If CONFIG_KUNIT=n (or m), this compiles away to nothing. If
> CONFIG_KUNIT=y, it will compile down to a NOP (on most architectures) if
> no KUnit test is currently running.
>
> Note that kunit_get_current_test() does not work if KUnit is built as a
> module. This mirrors the existing restriction on kunit_fail_current_test().
>
> Note that the definition of kunit_fail_current_test() still wraps an
> empty, inline function if KUnit is not built-in. This is to ensure that
> the printf format string __attribute__ will still work.
>
> Also update the documentation to suggest users use the new
> kunit_get_current_test() function, update the example, and to describe
> the behaviour when KUnit is disabled better.
>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Sadiya Kazi <sadiyakazi@...gle.com>
> Signed-off-by: David Gow <davidgow@...gle.com>
> Reviewed-by: Daniel Latypov <dlatypov@...gle.com>
> ---
Thank you, David. This looks great to me.
Reviewed-by: Sadiya Kazi <sadiyakazi@...gle.com>

Best Regards,
Sadiya Kazi
>
> As-is, the only code which will be directly affected by this (via the
> kunit_fail_current_test() change) will be UBSAN's KUnit integration.
>
> Patches to port other tests to use kunit_get_current_test() will be sent
> separately (other than the SLUB one in patch 3/3). KASAN in particular
> are reworking their KUnit tests and integration, so we'll use this in a
> follow up to avoid introducing a conflict.
>
> Changes since v3:
> https://lore.kernel.org/linux-kselftest/20221119081252.3864249-2-davidgow@google.com/
> - Use DECLARE_STATIC_KEY_FALSE() -- Thanks Daniel.
> - Some documentation rewording to make the behaviour a bit clearer.
>   - Thanks Daniel and Sadiya
>
> Changes since v2:
> https://lore.kernel.org/all/20221025071907.1251820-2-davidgow@google.com/
> - Only add kunit_get_current_test() when KUnit is built-in, as the
>   static key isn't available otherwise.
>   - I'm going to try to put together some patches to make things like
>     this available when CONFIG_KUNIT=m in the future.
>   - Also update the documentation to note this.
>
> Changes since v1:
> https://lore.kernel.org/linux-kselftest/20221021072854.333010-2-davidgow@google.com/
> - Fix a missing '}' which broke everything. Thanks Kees, kernel test
>   robot.
> - Add the new kunit_get_current_test() function, as most of the cases
>   where we retrieve the current test (even to fail it) were accessing
>   current->kunit_test directly, not using kunit_fail_current_test().
> - Add some documentation comments.
> - Update the documentation in usage.rst.
>   - The version in tips.rst was not updated, and will be removed:
>   https://lore.kernel.org/linux-kselftest/20221025055844.1231592-1-davidgow@google.com/
>
> ---
>  Documentation/dev-tools/kunit/usage.rst | 30 +++++++++-----
>  include/kunit/test-bug.h                | 53 +++++++++++++++++++++++--
>  2 files changed, 71 insertions(+), 12 deletions(-)
>
> diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
> index 22416ebb94ab..48f8196d5aad 100644
> --- a/Documentation/dev-tools/kunit/usage.rst
> +++ b/Documentation/dev-tools/kunit/usage.rst
> @@ -641,17 +641,23 @@ as shown in next section: *Accessing The Current Test*.
>  Accessing The Current Test
>  --------------------------
>
> -In some cases, we need to call test-only code from outside the test file.
> -For example, see example in section *Injecting Test-Only Code* or if
> -we are providing a fake implementation of an ops struct. Using
> -``kunit_test`` field in ``task_struct``, we can access it via
> -``current->kunit_test``.
> +In some cases, we need to call test-only code from outside the test file.  This
> +is helpful, for example, when providing a fake implementation of a function, or
> +to fail any current test from within an error handler.
> +We can do this via the ``kunit_test`` field in ``task_struct``, which we can
> +access using the ``kunit_get_current_test()`` function in ``kunit/test-bug.h``.
>
> -The example below includes how to implement "mocking":
> +``kunit_get_current_test()`` is safe to call even if KUnit is not enabled. If
> +KUnit is not enabled, was built as a module (``CONFIG_KUNIT=m``), or no test is
> +running in the current task, it will return ``NULL``. This compiles down to
> +either a no-op or a static key check, so will have a negligible performance
> +impact when no test is running.
> +
> +The example below uses this to implement a "mock" implementation of a function, ``foo``:
>
>  .. code-block:: c
>
> -       #include <linux/sched.h> /* for current */
> +       #include <kunit/test-bug.h> /* for kunit_get_current_test */
>
>         struct test_data {
>                 int foo_result;
> @@ -660,7 +666,7 @@ The example below includes how to implement "mocking":
>
>         static int fake_foo(int arg)
>         {
> -               struct kunit *test = current->kunit_test;
> +               struct kunit *test = kunit_get_current_test();
>                 struct test_data *test_data = test->priv;
>
>                 KUNIT_EXPECT_EQ(test, test_data->want_foo_called_with, arg);
> @@ -691,7 +697,7 @@ Each test can have multiple resources which have string names providing the same
>  flexibility as a ``priv`` member, but also, for example, allowing helper
>  functions to create resources without conflicting with each other. It is also
>  possible to define a clean up function for each resource, making it easy to
> -avoid resource leaks. For more information, see Documentation/dev-tools/kunit/api/test.rst.
> +avoid resource leaks. For more information, see Documentation/dev-tools/kunit/api/resource.rst.
>
>  Failing The Current Test
>  ------------------------
> @@ -719,3 +725,9 @@ structures as shown below:
>         static void my_debug_function(void) { }
>         #endif
>
> +``kunit_fail_current_test()`` is safe to call even if KUnit is not enabled. If
> +KUnit is not enabled, was built as a module (``CONFIG_KUNIT=m``), or no test is
> +running in the current task, it will do nothing. This compiles down to either a
> +no-op or a static key check, so will have a negligible performance impact when
> +no test is running.
> +
> diff --git a/include/kunit/test-bug.h b/include/kunit/test-bug.h
> index 5fc58081d511..c1b2e14eab64 100644
> --- a/include/kunit/test-bug.h
> +++ b/include/kunit/test-bug.h
> @@ -9,16 +9,63 @@
>  #ifndef _KUNIT_TEST_BUG_H
>  #define _KUNIT_TEST_BUG_H
>
> -#define kunit_fail_current_test(fmt, ...) \
> -       __kunit_fail_current_test(__FILE__, __LINE__, fmt, ##__VA_ARGS__)
> -
>  #if IS_BUILTIN(CONFIG_KUNIT)
>
> +#include <linux/jump_label.h> /* For static branch */
> +#include <linux/sched.h>
> +
> +/* Static key if KUnit is running any tests. */
> +DECLARE_STATIC_KEY_FALSE(kunit_running);
> +
> +/**
> + * kunit_get_current_test() - Return a pointer to the currently running
> + *                           KUnit test.
> + *
> + * If a KUnit test is running in the current task, returns a pointer to its
> + * associated struct kunit. This pointer can then be passed to any KUnit
> + * function or assertion. If no test is running (or a test is running in a
> + * different task), returns NULL.
> + *
> + * This function is safe to call even when KUnit is disabled. If CONFIG_KUNIT
> + * is not enabled, it will compile down to nothing and will return quickly no
> + * test is running.
> + */
> +static inline struct kunit *kunit_get_current_test(void)
> +{
> +       if (!static_branch_unlikely(&kunit_running))
> +               return NULL;
> +
> +       return current->kunit_test;
> +}
> +
> +
> +/**
> + * kunit_fail_current_test() - If a KUnit test is running, fail it.
> + *
> + * If a KUnit test is running in the current task, mark that test as failed.
> + *
> + * This macro will only work if KUnit is built-in (though the tests
> + * themselves can be modules). Otherwise, it compiles down to nothing.
> + */
> +#define kunit_fail_current_test(fmt, ...) do {                                 \
> +               if (static_branch_unlikely(&kunit_running)) {                   \
> +                       __kunit_fail_current_test(__FILE__, __LINE__,           \
> +                                                 fmt, ##__VA_ARGS__);          \
> +               }                                                               \
> +       } while (0)
> +
> +
>  extern __printf(3, 4) void __kunit_fail_current_test(const char *file, int line,
>                                                     const char *fmt, ...);
>
>  #else
>
> +static inline struct kunit *kunit_get_current_test(void) { return NULL; }
> +
> +/* We define this with an empty helper function so format string warnings work */
> +#define kunit_fail_current_test(fmt, ...) \
> +               __kunit_fail_current_test(__FILE__, __LINE__, fmt, ##__VA_ARGS__)
> +
>  static inline __printf(3, 4) void __kunit_fail_current_test(const char *file, int line,
>                                                             const char *fmt, ...)
>  {
> --
> 2.38.1.584.g0f3c55d4c2-goog
>
> --
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