* Pavel Machek <pavel@xxxxxx> 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));
> }
That's the old 32-bit x86 trick to compute 'current' from the kernel stack
pointer.
It can be done better - for example on platforms with optimized percpu
variables (x86-64) it looks like this:
static inline struct thread_info *current_thread_info(void)
{
struct thread_info *ti;
ti = (void *)(this_cpu_read_stable(kernel_stack) +
KERNEL_STACK_OFFSET - THREAD_SIZE);
return ti;
}
Which turns the computation of 'current' into a single instruction. For
example, to access current->pid [which fields is at offset 0x2a4], we get:
3ad0: 65 48 8b 04 25 00 00 mov %gs:0x0,%rax
3ad7: 00 00
3ad9: 8b 80 a4 02 00 00 mov 0x2a4(%rax),%eax
I also agree with the removal of the 'tsk' parameter because the function
itself internally assumes that tsk == current.
[ We could perhaps rename the function to
debug_check_no_locks_held_curr(), to make it clear it operates on the
current task. ]
Thanks,
Ingo
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