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SubjectRe: [PATCH tip/core/rcu 20/23] rcu: Remove callback acceleration from grace-period initialization
On Thu, Aug 30, 2012 at 11:18:35AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>
> Before grace-period initialization was moved to a kthread, the CPU
> invoking this code would have at least one callback that needed
> a grace period, often a newly registered callback. However, moving
> grace-period initialization means that the CPU with the callback
> that was requesting a grace period is not necessarily the CPU that
> is initializing the grace period, so this acceleration is less
> valuable. Because it also adds to the complexity of reasoning about
> correctness, this commit removes it.
>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> kernel/rcutree.c | 19 -------------------
> 1 files changed, 0 insertions(+), 19 deletions(-)
>
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 86903df..44609c3 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1055,25 +1055,6 @@ static int rcu_gp_init(struct rcu_state *rsp)
> rsp->gpnum++;
> trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
> record_gp_stall_check_time(rsp);
> -
> - /*
> - * Because this CPU just now started the new grace period, we
> - * know that all of its callbacks will be covered by this upcoming
> - * grace period, even the ones that were registered arbitrarily
> - * recently. Therefore, advance all RCU_NEXT_TAIL callbacks
> - * to RCU_NEXT_READY_TAIL. When the CPU later recognizes the
> - * start of the new grace period, it will advance all callbacks
> - * one position, which will cause all of its current outstanding
> - * callbacks to be handled by the newly started grace period.
> - *
> - * Other CPUs cannot be sure exactly when the grace period started.
> - * Therefore, their recently registered callbacks must pass through
> - * an additional RCU_NEXT_READY stage, so that they will be handled
> - * by the next RCU grace period.
> - */
> - rdp = __this_cpu_ptr(rsp->rda);
> - rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
> -
> raw_spin_unlock_irqrestore(&rnp->lock, flags);
>
> /* Exclude any concurrent CPU-hotplug operations. */
> --
> 1.7.8
>


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