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SubjectRe: check && lockdep_no_validate (Was: lockdep: Introduce wait-type checks)
On Thu, Jan 09, 2014 at 06:54:48PM +0100, Oleg Nesterov wrote:
> But this connects to lockdep_no_validate. Not sure I understand what
> this class should actually do, but consider this code:
>
> DEFINE_MUTEX(m1);
> DEFINE_MUTEX(m2);
> DEFINE_MUTEX(mx);
>
> void lockdep_should_complain(void)
> {
> lockdep_set_novalidate_class(&mx);
>
> // m1 -> mx -> m2
> mutex_lock(&m1);
> mutex_lock(&mx);
> mutex_lock(&m2);
> mutex_unlock(&m2);
> mutex_unlock(&mx);
> mutex_unlock(&m1);
>
>
> // m2 -> m1 ; should trigger the warning
> mutex_lock(&m2);
> mutex_lock(&m1);
> mutex_unlock(&m1);
> mutex_unlock(&m2);
> }
>
> lockdep doesn't not detect the trivial possible deadlock.
>
> The patch below seems to work but most probably it is not right, and
> I forgot everything (not too much) I knew about lockdep internals.
>
> Oleg.
>
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1939,7 +1939,8 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
> * Only non-recursive-read entries get new dependencies
> * added:
> */
> - if (hlock->read != 2) {
> + if (hlock->read != 2 &&
> + hlock->instance->key != &__lockdep_no_validate__) {
> if (!check_prev_add(curr, hlock, next,
> distance, trylock_loop))
> return 0;
>

Hmm, you are quite right indeed; although I would write it like:

if (hlock->read != 2 && hlock->check == 2)

because the __lockdep_no_validate__ thing forces the ->check value to 1.


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