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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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