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    SubjectRe: [GIT PULL rcu/next] fixes and breakup of memory-barrier-decrease patch
    On Sat, May 21, 2011 at 04:28:44PM +0200, Ingo Molnar wrote:
    > * Paul E. McKenney <> wrote:
    > > Hello, Ingo,
    > >
    > > This pull requests covers some RCU bug fixes and one patch rework.
    > >
    > > The first group breaks up the infamous now-reverted (but ultimately
    > > vindicated) "Decrease memory-barrier usage based on semi-formal proof"
    > > commit into five commits. These five commits immediately follow the
    > > revert, and the diff across all six of these commits is empty, so that
    > > the effect of the five commits is to revert the revert.
    > But ... the regression that was observed with that commit needs to be fixed
    > first, or not? In what way was the barrier commit vindicated?

    From what I can see, the hang was fixed by Frederic's patch at I was interpreting that as
    vindication, perhaps ill-advisedly.

    Yinghai said that he was still seeing a delay, adn that he was seeing
    it even with the "Decrease memory-barrier usage based on semi-formal
    proof" reverted: This hang seems
    to happen when he uses gcc 4.5.0, but not when using gcc 4.5.1, assuming
    I understood his sequence of emails. So I was interpreting that as
    meaning that the delay was unlikely to be caused by that commit, probably
    by one of the later commits.

    I clearly need to figure out what is causing this delay. I asked Yinghai
    to apply c7a378603 (Remove waitqueue usage for cpu, node, and boost kthreads)
    from Peter Zijlstra because the long delays that Yinghai is seeing
    (93 seconds for memory_dev_init() rather than 3 or 4 seconds) might be
    due to my less-efficient method of awakening the RCU kthreads, so that
    Peter's approache might help.

    If that doesn't speed things up for Yinghai, then I will work out some
    tracing to help localize the slowdown that he is seeing.

    Of course, if you would rather that I get to the bottom of this before
    pulling, fair enough!

    Thanx, Paul

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