On 11/18, Oleg Nesterov wrote:
>
> On 11/11, Oleg Nesterov wrote:
> >
> It turns out, lockdep annotations are not that simple due to internal
> locks used by percpu_rw_semaphore. To clarify, it is actually simple
> but lockdep_set_novalidate_class() doesn't seem to actually work, and
> more importantly, it must not be used according to checkpatch.pl.
Still, is __lockdep_no_validate__ logic correct? I am just curious.
Consider the following code,
DEFINE_MUTEX(m1);
DEFINE_MUTEX(m2);
DEFINE_MUTEX(mx);
static void trigger_lockdep_bug(bool novalidate)
{
if (novalidate)
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);
}
trigger_lockdep_bug(false) works correctly, but novalidate => true
confuses (I think) lockdep and it doesn't detect the trivial deadlock.
check_prev_add(m1, mx) still adds the new dependency, but then it is
ignored because of __lockdep_no_validate__ check.
Certainly I do not understand this code (and I am sure I will never
understand it even if I try ;) But perhaps something like below makes
sense? Or I misunderstood the purpose of lockdep_set_novalidate_class?
Thanks,
Oleg.
--- x/kernel/lockdep.c
+++ x/kernel/lockdep.c
@@ -1935,7 +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;
--
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