Re: [PATCH] locking/lockdep: Add debug_locks check in __lock_downgrade()

[Tetsuo Handa]

On 2019/01/10 19:21, Dmitry Vyukov wrote:
>> @@ -3535,6 +3535,9 @@ static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip)
>>         unsigned int depth;
>>         int i;
>>
>> +       if (unlikely(!debug_locks))
>> +               return 0;
>> +
> 
> Are we sure this resolves the problem rather than makes the
> inconsistency window smaller?

As far as I know, this should resolve the problem.

> I don't understand all surrounding code, but looking just at this
> function it looks like it may just pepper over the problem. Say, we
> pass this check when lockdep was still turned on. Then this thread is
> preempted for some time (e.g. a virtual CPU), then another thread
> started reporting a warning, turned lockdep off, some information
> wasn't collected, and this this task resumes and reports a false
> warning.

What this function checks is whether current thread is holding rw_semaphore
for write. Since the information of held locks are per "struct task_struct"
record, if lockdep is still enabled as of entry of this function, there must
be a lockdep record that current thread is holding rw_semaphore for write
if current thread is actually holding rw_semaphore for write. Therefore,
preemption/interrupts can't erase the lockdep record that current thread is
holding rw_semaphore, even if lockdep is turned off after passing this check.

> Or we are holding the mutex here, and the fact that we are holding it
> ensures that no other task will take it and no information will be
> lost?
> Quite a tricky moment, perhaps deserves a comment.

I think many other functions check debug_locks upon entry of functions.

> 
>>         depth = curr->lockdep_depth;
>>         /*
>>          * This function is about (re)setting the class of a held lock,