taskqueue_block

[John Baldwin]

On Tuesday, November 19, 2013 10:29:18 pm Adrian Chadd wrote:
> Yes, and lets fix this. :)

Hmm, is taskqueue_block() always used in context where waiting is safe?
 
> On 19 November 2013 06:32, Andriy Gapon <avg at freebsd.org> wrote:
> >
> > Forwarding this to the larger audience for a discussion.
> >
> > -------- Original Message --------
> > Message-ID: <5287BDB9.10201 at FreeBSD.org>
> > Date: Sat, 16 Nov 2013 20:47:21 +0200
> > From: Andriy Gapon <avg at FreeBSD.org>
> > Subject: taskqueue_block
> >
> >
> >
> > It seems that either I do not understand something about taskqueue_block code or
> > it is a quite dangerous and abused API.  The fact that it is not properly
> > documented does not help either.
> >
> > The commit message said:
> >> Implement taskqueue_block() and taskqueue_unblock(). These functions allow the
> >> owner of a queue to block and unblock execution of the tasks in the queue while
> >> allowing tasks to continue to be added queue. Combining this with
> >> taskqueue_drain() allows a queue to be safely disabled. The unblock function may
> > [...]
> >
> > I indeed see this (anti?) pattern being used in the code.
> > But what about the following case.   One thread calls taskqueue_block() and sets
> > TQ_FLAGS_BLOCKED.  Another thread calls taskqueue_enqueue, this adds a task to
> > the queue and sets ta_pending of the task to 1.  tq_enqueue is not called, so an
> > actual queue runner is not called or waken up.   Then the first thread calls
> > taskqueue_drain() on the task.  As far as I can see, the thread would then just
> > wait forever because the task is pending and is not going to be executed.
> >
> > Additionally, it is impossible to reason about the taskqueue's state after
> > taskqueue_block call, because the call just sets the flag and does not do any
> > synchronization.  And as described above, it is not safe to call APIs that could
> > allow the taskqueue or the task state to become known.
> >
> > I think that taskqueue_block() should wait on the currently active tasks to
> > complete.  I don't think that this behavior could be optional.  I do see any
> > reasonable and safe use for "non-blocking" taskqueue_block().
> > taskqueue_drain() calls after taskqueue_block() must be removed.  The code
> > should either use taskqueue_drain() or "blocking" taskqueue_block() depending on
> > concrete circumstances.
> >
> > What do you think?
> > Thank you.
> > --
> > Andriy Gapon
> >
> >
> >
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-- 
John Baldwin