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    SubjectRe: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12
    On Wed, Apr 19, 2017 at 01:48:08PM +0200, Christian Borntraeger wrote:
    > On 04/19/2017 01:28 PM, Peter Zijlstra wrote:
    > >
    > > So the thing Maz complained about is because KVM assumes
    > > synchronize_srcu() is 'free' when there is no srcu_read_lock() activity.
    > > This series 'breaks' that.
    >
    > Why is such a behaviour change not mentioned in the cover letter?
    > I could not find anything in the patch descriptions that would
    > indicate a slowdown. How much slower did it get?
    >
    > But indeed, there are several places at KVM startup which have been
    > reworked to srcu since normal rcu was too slow for several usecases.
    > (Mostly registering devices and related data structures at startup,
    > basically the qemu/kvm coldplug interaction)

    I suspect Paul is not considering this a 'normal' RCU feature, and
    therefore didn't think about changing this.

    I know I was fairly surprised by this requirement when I ran into it;
    and only accidentally remembered it now that maz complained.

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