Messages in this thread | | | Date | Thu, 7 Dec 2006 08:54:09 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH 3/3] WorkStruct: Use direct assignment rather than cmpxchg() |
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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.
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