Re: [PATCH v4] lockdep: check that no locks held at freeze time

[Mandeep Singh Baines]

On Wed, Feb 20, 2013 at 3:24 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 20 Feb 2013 15:17:16 -0800
> Mandeep Singh Baines <msb@xxxxxxxxxxxx> wrote:
>
>> We shouldn't try_to_freeze if locks are held.
>>
>> ...
>>
>> @@ -43,6 +44,9 @@ extern void thaw_kernel_threads(void);
>>
>> +     if (!(current->flags & PF_NOFREEZE))
>> +             debug_check_no_locks_held(current,
>> +
>>                                         "lock held while trying to freeze");
>> ...
>>
>> +     debug_check_no_locks_held(tsk, "lock held at task exit time");
>
> There doesn't seem much point in adding the `msg' to
> debug_check_no_locks_held() - the dump_stack() in
> print_held_locks_bug() will tell us the same thing.  Maybe just change

dump_stack() can be confusing when there is inlining. On occasion I've
looked at the wrong mutex_lock, for example, when there was another
mutex_lock that was inlined. Of course, you can start objdump and
verify the offsets. But that requires that you have the object file.
You could have a try_to_freeze added to do_exit. I was thinking of
adding another locks_held in the return from syscall path.

How about if we did some inlining and printed out the function, file
and line number where the check was placed:

#define debug_check_no_locks_held() do { \
        if (unlikely(current->lockdep_depth > 0)) { \
                printk("BUG: locks helds at %s:%d/%s()!\n", __FILE__,
__LINE__, __func__); \
                print_held_locks_bug(); \
        } \
} while (0)

That we avoid any potential confusion.

> the print_held_locks_bug() messages so they stop assuming they were
> called from do_exit()?
>
> Also, I wonder if the `tsk' arg is needed.  In both callers
> tsk==current.  Is it likely that we'll ever call
> debug_check_no_locks_held() for any task other than `current'?
>

I agree. I'll add that change to the patch once we decide what to about msg.

Regards,
Mandeep
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