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

[Mandeep Singh Baines]

On Wed, Feb 20, 2013 at 4:20 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 20 Feb 2013 16:17:39 -0800
> Mandeep Singh Baines <msb@xxxxxxxxxxxx> wrote:
>
>> 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.
>
> Backtraces aren't *that* bad.  We'll easily be able to tell which of
> the two callsites triggered the trace.
>

Let's say there was a try_to_freeze() that got inlined indirectly
(multiple levels of inline) into do_exit. Wouldn't the backtraces for
the regular exit check and the try_to_freeze check be identical except
for the offset (do_exit+0x45 versus do_exit+0x88)? So unless you had
an object file you wouldn't know which check you hit.

Regards,
Mandeep
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