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SubjectRe: [PATCH tip/core/rcu 12/15] rcu: Remove redundant memory barrier from __call_rcu()
On Thu, Aug 30, 2012 at 11:56:25AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@linaro.org>
>
> The first memory barrier in __call_rcu() is supposed to order any
> updates done beforehand by the caller against the actual queuing
> of the callback. However, the second memory barrier (which is intended
> to order incrementing the queue lengths before queuing the callback)
> is also between the caller's updates and the queuing of the callback.
> The second memory barrier can therefore serve both purposes.
>
> This commit therefore removes the first memory barrier.

I don't see any such second memory barrier in __call_rcu(), at least not
in current master. Right after this smp_mb(), __call_rcu() enqueues the
callback and increments the queue length.

Did you add a second memory barrier in some other patch that hasn't made
it upstream yet? If so, could you note that patch dependency explicitly
in the commit message?

- Josh Triplett

> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> kernel/rcutree.c | 2 --
> 1 files changed, 0 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index e58097b..5b6709b 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1923,8 +1923,6 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
> head->func = func;
> head->next = NULL;
>
> - smp_mb(); /* Ensure RCU update seen before callback registry. */
> -
> /*
> * Opportunistically note grace-period endings and beginnings.
> * Note that we might see a beginning right after we see an
> --
> 1.7.8
>


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