Re: Lots of bugs with current->state = TASK_*INTERRUPTIBLE

David Daney
2010-01-21T19:18:50Z

Steven Rostedt wrote:
> Peter Zijlstra and I were doing a look over of places that assign
> current->state = TASK_*INTERRUPTIBLE, by simply looking at places with:
>
> $ git grep -A1 'state[[:space:]]*=[[:space:]]*TASK_[^R]'
>
> and it seems there are quite a few places that looks like bugs. To be on
> the safe side, everything outside of a run queue lock that sets the
> current state to something other than TASK_RUNNING (or dead) should be
> using set_current_state().
>
> current->state = TASK_INTERRUPTIBLE;
> schedule();
>
> is probably OK, but it would not hurt to be consistent. Here's a few
> examples of likely bugs:
>
[...]

This may be a bit off topic, but exactly which type of barrier should
set_current_state() be implying?

On MIPS, set_mb() (which is used by set_current_state()) has a full mb().

Some MIPS based processors have a much lighter weight wmb(). Could
wmb() be used in place of mb() here?

If not, an explanation of the required memory ordering semantics here
would be appreciated.

I know the documentation says:

set_current_state() includes a barrier so that the write of
current->state is correctly serialised wrt the caller's subsequent
test of whether to actually sleep:

set_current_state(TASK_UNINTERRUPTIBLE);
if (do_i_need_to_sleep())
schedule();

Since the current CPU sees the memory accesses in order, what can be
happening on other CPUs that would require a full mb()?

Thanks,
David Daney