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SubjectRe: [PATCH 3/3] WorkStruct: Use direct assignment rather than cmpxchg()
On Thu, 07 Dec 2006 15:31:43 +0000
David Howells <dhowells@redhat.com> wrote:

> Use direct assignment rather than cmpxchg() as the latter is unavailable and
> unimplementable on some platforms and is actually unnecessary.
>
> The use of cmpxchg() was to guard against two possibilities, neither of which
> can actually occur:
>
> (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, 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.
>
> If that's a problem, then a bitops-proofed assignment will be required -
> equivalent to atomic_set() vs other atomic_xxx() ops.
>

I don't understand, as usual.

afacit in all (but one) cases we do

if (!test_and_set_bit(WORK_STRUCT_PENDING, &work->management)) {
...
set_wq_data(work, wq);
...
<now do stuff which makes it possible for run_workqueue()
to get a look at the new work>
}

cancel_delayed_work() looks OK too.

The possible exception is schedule_on_each_cpu() which is being lazy, but
looks fixable.


So... afaict in all the places where there can be a concurrent
set_wq_data() and test_and_set_bit(), WORK_STRUCT_PENDING is reliably set,
and we can assume (and ensure) that a failing test_and_set_bit() will not
write to the affected word at all.

What am I missing?
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