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


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