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    SubjectRe: cmpxchg() in kernel/workqueue.c breaks things
    On Thu, 07 Dec 2006 11:03:49 +0000
    David Howells <dhowells@redhat.com> wrote:

    > David Miller <davem@davemloft.net> wrote:
    >
    > > David, you have to fix the locking scheme used in kernel/workqueue.c,
    > > you absolutely cannot assume that cmpxchg() is available on all
    > > platforms. This breaks the build on the platforms that don't
    > > have such an instruction, and no it cannot emulated.
    >
    > Yeah, I've figured that one out. Also, having considered things last night, I
    > think the use of cmpxchg() is unnecessary.
    >
    > I was trying to handle against two possibilities:
    >
    > (1) The pending flag may have been unset or may be cleared. However, given
    > where it's called, the pending flag is _always_ set. I don't think it
    > can be unset whilst we're in set_wq_data().
    >
    > Once the work is enqueued to be actually run, the only way off the queue
    > is for it to be actually run.
    >
    > If it's a delayed work item, then the bit can't be cleared by the timer
    > because we haven't started the timer yet. Also, the pending bit can't be
    > cleared by cancelling the delayed work _until_ the work item has had its
    > timer started.
    >
    > (2) The workqueue pointer might change. This can only happen in two cases:
    >
    > (a) The work item has just been queued to actually run, and so we're
    > protected by the appropriate workqueue spinlock.
    >
    > (b) A delayed work item is being queued, and so the timer hasn't been
    > started yet, and so no one else knows about the work item or can
    > access it (the pending bit protects us).
    >
    > Besides, set_wq_data() _sets_ the workqueue pointer unconditionally, so
    > it can be assigned instead.
    >
    > So, I think replacing the set_wq_data() with a straight assignment would be
    > okay in most cases. The problem is where we end up tangling with
    > test_and_set_bit() emulated using spinlocks, and even then it's not a problem
    > _provided_ test_and_set_bit() doesn't attempt to modify the word if the bit
    > was set.
    >

    I don't see why the 2.6.19 logic needed changing.

    a) Nobody should be freeing the work_struct itself without running
    flush_scheduled_work() and

    b) even if the work_struct _did_ get freed, the callback function won't
    care, because there's nothing in that work_struct which it's interested
    in.


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