linux-kernel - Re: [PATCH] kunit: added lockdep support

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Date:   Thu, 6 Aug 2020 15:43:39 -0500
From:   Uriel Guajardo <urielguajardo@...gle.com>
To:     Uriel Guajardo <urielguajardojr@...il.com>
Cc:     Brendan Higgins <brendanhiggins@...gle.com>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kunit: added lockdep support

On Thu, Aug 6, 2020 at 3:37 PM Uriel Guajardo <urielguajardojr@...il.com> wrote:
>
> From: Uriel Guajardo <urielguajardo@...gle.com>
>
> KUnit tests will now fail if lockdep detects an error during a test
> case.
>
> The idea comes from how lib/locking-selftest [1] checks for lock errors: we
> first if lock debugging is turned on. If not, an error must have
> occurred, so we fail the test and restart lockdep for the next test case.
>
> Like the locking selftests, we also fix possible preemption count
> corruption from lock bugs.
>
> Depends on kunit: support failure from dynamic analysis tools [2]
>
> [1] https://elixir.bootlin.com/linux/v5.7.12/source/lib/locking-selftest.c#L1137
>
> [2] https://lore.kernel.org/linux-kselftest/20200806174326.3577537-1-urielguajardojr@gmail.com/
>
> Signed-off-by: Uriel Guajardo <urielguajardo@...gle.com>
> ---
>  lib/kunit/test.c | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/lib/kunit/test.c b/lib/kunit/test.c
> index d8189d827368..0838ececa005 100644
> --- a/lib/kunit/test.c
> +++ b/lib/kunit/test.c
> @@ -11,6 +11,8 @@
>  #include <linux/kref.h>
>  #include <linux/sched/debug.h>
>  #include <linux/sched.h>
> +#include <linux/lockdep.h>
> +#include <linux/debug_locks.h>
>
>  #include "debugfs.h"
>  #include "string-stream.h"
> @@ -22,6 +24,26 @@ void kunit_fail_current_test(void)
>                 kunit_set_failure(current->kunit_test);
>  }
>
> +static inline void kunit_check_locking_bugs(struct kunit *test,
> +                                           unsigned long saved_preempt_count)
> +{
> +       preempt_count_set(saved_preempt_count);
> +#ifdef CONFIG_TRACE_IRQFLAGS
> +       if (softirq_count())
> +               current->softirqs_enabled = 0;
> +       else
> +               current->softirqs_enabled = 1;
> +#endif

I am not entirely sure why lib/locking-selftests enables/disables
softirqs, but I suspect it has to do with the fact that preempt_count
became corrupted, and somehow softirqs became incorrectly
enabled/disabled as a result. The resetting of the preemption count
will undo the enabling/disabling accordingly. Any insight on this
would be appreciated!

> +#if IS_ENABLED(CONFIG_LOCKDEP)
> +       local_irq_disable();
> +       if (!debug_locks) {
> +               kunit_set_failure(test);
> +               lockdep_reset();
> +       }
> +       local_irq_enable();
> +#endif
> +}
> +
>  static void kunit_print_tap_version(void)
>  {
>         static bool kunit_has_printed_tap_version;
> @@ -289,6 +311,7 @@ static void kunit_try_run_case(void *data)
>         struct kunit *test = ctx->test;
>         struct kunit_suite *suite = ctx->suite;
>         struct kunit_case *test_case = ctx->test_case;
> +       unsigned long saved_preempt_count = preempt_count();
>
>         current->kunit_test = test;
>
> @@ -298,7 +321,8 @@ static void kunit_try_run_case(void *data)
>          * thread will resume control and handle any necessary clean up.
>          */
>         kunit_run_case_internal(test, suite, test_case);
> -       /* This line may never be reached. */
> +       /* These lines may never be reached. */
> +       kunit_check_locking_bugs(test, saved_preempt_count);
>         kunit_run_case_cleanup(test, suite);
>  }
>
> --
> 2.28.0.236.gb10cc79966-goog
>