Re: [PATCH] Add kerneldoc for flush_scheduled_work()

Alan Stern
2009-08-12T17:25:24Z

On Wed, 12 Aug 2009, James Bottomley wrote:

> > > 2. If you have to hold a lock while waiting:
> > > 1. If it's a global lock, make sure you're using a local
> > > queue and that nothing you submitted to the queue can
> > > take the lock
> > > 2. If it's a local lock, you may use a global queue but
> > > must still make sure that nothing you submitted to the
> > > queue can take the lock.

> > Anyway, 2.1 and 2.2 are wrong. They should read: "... make sure that

(I overstated things; 2.1 is okay. But 2.2. is wrong. And
flush_scheduled_work() definitely concerns a global queue.)

> > nothing submitted to the queue calls any of your routines that can take
> > the lock." The point being that even though _you_ don't submit
> > anything bad to the queue, somebody else might do so.
>
> Semantically "make sure that nothing submitted to the queue can take a
> lock" means exactly that ... it includes all routines called by the
> queue function.

James, you wrote "make sure that nothing _you_ submitted to the queue"
(my emphasis). That is quite different from "make sure that nothing
submitted to the queue". One is doable, the other isn't.

> > If you use cancel_work_sync() instead of flush_scheduled_work() then
> > the rules become less onerous. In place of your 2.1 and 2.2, we have:
> >
> > Make sure the work item you are cancelling cannot take
> > the lock.
> >
> > This is much easier to verify.
>
> It's the same, surely ... you must verify that everything going on to
> the queue obeys the rules otherwise you get things on it which can't be
> cancelled ...

Not at all. With cancel_work_sync() you must verify only that the
item you want to cancel obeys the rules. There's no need to check the
other items -- and a public workqueue like keventd_wq certainly will
contain other items.

> and thus either the advice to call cancel is wrong, or we
> could call flush because the locking rules are obeyed.

No; as shown above, your logic is flawed.

Alan Stern

P.S.: Ingo, I rather expected to see you comment on this thread. What
is your opinion?