Roland Dreier <roland@topspin.com> wrote:
>
> Still, I like the idea of this patch, since it resolves the livelock.
> But I don't think the implementation is quite right. insert_sequence
> doesn't get incremented until delayed_work_timer_fn(). That means
> that a driver (tty_io.c, for example) could call
> schedule_delayed_work(), then call flush_scheduled_work() before
> delayed_work_timer_fn() has run for that work.
The driver needs to run cancel_delayed_work() before calling
flush_scheduled_work(). The tty patch is already doing that, and I think
that plugs the holes.
Here's a full changelog.
The workqueue code currently has a notion of a per-cpu queue being "busy".
flush_scheduled_work()'s responsibility is to wait for a queue to be not busy.
Problem is, flush_scheduled_work() can easily hang up.
- The workqueue is deemed "busy" when there are pending (timer-based)
works. But if someone repeatedly schedules new work in the delayed
callback, the queue will never fall idle, and flush_scheduled_work() will
not complete.
- If someone reschedules work (not delayed work) in the work function, that
too will cause the queue to never go idle, and flush_scheduled_work() will
not terminate.
So what this patch does is:
- Create a new "cancel_delayed_work()" which will try to kill off any
timer-based works.
- Change flush_scheduled_work() so that it is immune to people re-adding
work in the work callout handler.
We can do this by recognising that the caller does *not* want to wait
until the workqueue is "empty". The caller merely wants to wait until all
works which were pending at the time flush_scheduled_work() was called have
completed.
The patch uses a couple of sequence numbers for that.
So now, if someone wants to reliably remove delayed work they should do:
/*
* Make sure that my work-callback will no longer schedule new work
*/
my_driver_is_shutting_down = 1;
/*
* Kill off any pending delayed work
*/
cancel_delayed_work(&my_work);
/*
* OK, there will be no new works scheduled. But there may be one
* currently queued or in progress. So wait for that to complete.
*/
flush_scheduled_work();
And change the flush_workqueue() sleep to be uninterruptible. We worry that
delivery of a signal may cause the wait to return too early.
-
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This archive was generated by hypermail 2b29 : Tue Apr 15 2003 - 22:00:17 EST