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SubjectRe: [PATCH] floppy: convert to delayed work and single-thread wq
On Wed, May 16, 2012 at 10:44:31PM +0200, Jiri Kosina wrote:
> On Wed, 16 May 2012, Tejun Heo wrote:
>
> > > In floppy_grab_irq_and_dma() the point is to drain the workqueue
> > > completely (before the conversion, we were just using
> > > flush_work_sync(&floppy_work) for particular work item), and for that
> > > flush_work() is not sufficient any more.
> > >
> > > So I am really considering going back to driver-specific singlethreaded
> > > workqueue.
> >
> > Ummm... still confused. flush_work_sync() is fine too. If you have
> > two, two calls to flush_work_sync() are equivalent to flushing the
> > workqueue in effect. You just need to avoid flush_workqueue() because
> > system workqueues may be hosting work items which can run arbitrarily
> > long.
>
> Before the conversion, we do
>
> flush_work_sync(&floppy_work);
>
> in floppy_grab_irq_and_dma(). After the conversion, the single-threaded
> workqueue is used to queue more than just floppy_work, and we want all
> this to be flushed before proceeding, so neither flush_work() nor
> flush_work_sync() is enough, as there might be floppy_work, fd_timer or
> fd_timeout queued. This all has to be flushed.
>
> If this still doesn't seem to make sense, I'll get back to it tomorrow, it
> might be just too late and my brain cells might already be dreaming.

So, AFAICS, there are only three work items in question here,
floppy_work, fd_timer and fd_timeout, so three calls to
flush[_delayed]_work_sync() is functionally equivalent to
flush_workqueue(). While flushing separately may take longer, AFAICS,
it doesn't seem to be on the hot path (not much in floppy can be
considered hot these days tho) and I don't think it would make any
noticeable performance difference.

If having a separate flush domain is beneficial, please create a
non-reentrant workqueue without rescuer - alloc_workqueue("floppy",
WQ_NON_REENTRANT, 0). If serialization among different work items is
necessary, single thread workqueue without rescuer can be used -
alloc_workqueue("floppy", WQ_UNBOUND, 1).

create_*workqueue() are left there for backwards compatibility and
they all have rescuer for that reason. Gotta clean them up and maybe
create different wrappers, I guess.

Thanks.

--
tejun


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