Re: [PATCH 2/2] lockdep: check that no locks held at freeze time

[Colin Cross <ccross-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>]

On Sat, May 4, 2013 at 5:05 PM, Pavel Machek <pavel-+ZI9xUNit7I@public.gmane.org> wrote:
> Hi!
>
>> >> >> --- a/kernel/exit.c
>> >> >> +++ b/kernel/exit.c
>> >> >> @@ -835,7 +835,7 @@ void do_exit(long code)
>> >> >>       /*
>> >> >>        * Make sure we are holding no locks:
>> >> >>        */
>> >> >> -     debug_check_no_locks_held(tsk);
>> >> >> +     debug_check_no_locks_held();
>> >> >
>> >> > Is task guaranteed == current?
>> >>
>> >> Yes, the first line of do_exit is:
>> >>         struct task_struct *tsk = current;
>> >
>> > Aha, I understand it now.
>> >
>> > Accessing current is slower than local variable. So your "new" code
>> > will work but will be slower. Please revert this part.
>>
>> Using current instead of passing in tsk was done at Andrew Morton's
>> suggestion, and makes no difference from the freezer's perspective
>> since it would have to use current to get the task to pass in, so I'm
>> going to leave it as is.
>
> Well, current is:
>
> static inline struct thread_info *current_thread_info(void)
> {
>         register unsigned long sp asm ("sp");
>         return (struct thread_info *)(sp & ~(THREAD_SIZE - 1));
> }
>
> #define get_current() (current_thread_info()->task)
>
> #define current get_current()
>
> Instead of passing computed value to debug_check_no_locks_held(), you
> force it to be computed again. do_exit() performance matters for
> configure scripts, etc.
>
> I'd say it makes sense to keep the optimalization. akpm can correct
> me.

That translates to 3 instructions, with no memory accesses:
c0008350:       e1a0300d        mov     r3, sp
c0008354:       e3c32d7f        bic     r2, r3, #8128   ; 0x1fc0
c0008358:       e3c2203f        bic     r2, r2, #63     ; 0x3f
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