Re: [ 097/120] rcu: Fix day-one dyntick-idle stall-warning bug

Ben Hutchings
2012-10-12T22:14:44Z

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On Thu, 2012-10-11 at 10:00 +0900, Greg Kroah-Hartman wrote:
> 3.4-stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: "Paul E. McKenney" <paul.mckenney@linaro.org>
>
> commit a10d206ef1a83121ab7430cb196e0376a7145b22 upstream.
[...]
> This commit therefore makes CPUs check more carefully before starting a
> new grace period. This new check relies on an array of tail pointers
> into each CPU's list of callbacks. If the CPU is up to date on which
> grace periods have completed, it checks to see if any callbacks follow
> the RCU_DONE_TAIL segment, otherwise it checks to see if any callbacks
> follow the RCU_WAIT_TAIL segment. The reason that this works is that
> the RCU_WAIT_TAIL segment will be promoted to the RCU_DONE_TAIL segment
> as soon as the CPU is officially notified that the old grace period
> has ended.
[...]
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -295,7 +295,9 @@ cpu_has_callbacks_ready_to_invoke(struct
> static int
> cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
> {
> - return *rdp->nxttail[RCU_DONE_TAIL] && !rcu_gp_in_progress(rsp);
> + return *rdp->nxttail[RCU_DONE_TAIL +
> + ACCESS_ONCE(rsp->completed) != rdp->completed] &&

This is a very obscurely written expression. The array index is parsed
as:
(RCU_DONE_TAIL + ACCESS_ONCE(rsp->completed)) != rdp->completed

Since RCU_DONE_TAIL == 0 and RCU_WAIT_TAIL == 1, this is then equivalent
to:
ACCESS_ONCE(rsp->completed) != rdp->completed
or:
(ACCESS_ONCE(rsp->completed) != rdp->completed) ? RCU_WAIT_TAIL : RCU_DONE_TAIL

But whyever didn't you write that explicitly?

Ben.

> + !rcu_gp_in_progress(rsp);
> }
>
> /*

--
Ben Hutchings
Kids! Bringing about Armageddon can be dangerous. Do not attempt it in
your own home. - Terry Pratchett and Neil Gaiman, `Good Omens'

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