On 10/31, Peter Zijlstra wrote:
>
> Provide better implementations of wait_event_freezable() APIs.
>
> The problem is with freezer_do_not_count(), it hides the thread from
> the freezer, even though this thread might not actually freeze/sleep
> at all.
I agree, wait_event_freezable() is awful. But could you clarify "at all" ?
Sure, the task can be preempted right after it sets, it can do a lot
of things before it calls schedule(), it can be woken after that and
it can run again and do something else before freezer_count() calls
try_to_freeze(), etc.
Is this what you meant?
> +#define __wait_event_freezable(wq, condition)
> \
> + (void)___wait_event(wq, condition, TASK_INTERRUPTIBLE, 0, 0, \
> + schedule(); try_to_freeze())
I don't think this can work. wait_event_freezable() should be used by
kernel threads and thus we can't rely on TIF_SIGPENDING, freeze_task()
simply does wake_up_state(TASK_INTERRUPTIBLE) in this case.
Just for example, suppose that try_to_freeze_tasks() calls freeze_task()
before this kthread sets current->state = TASK_INTERRUPTIBLE. In this
case __wait_event_freezable()->schedule() will happily sleep and
try_to_freeze_tasks() will fail.
That is why I tried to suggest cmd == freezable_schedule(). Still not
good, but at least this narrows the window and (perhaps) we can improve
this freezable_schedule() later.
But on a second thought... Probably cmd => try_to_freeze(); schedule();
should work. Or just
#define __wait_event_freezable(wq, condition) \
__wait_event_interruptible(wq, ({ try_to_freeze(); (condition);
}))
which looks simpler.
Oleg.
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