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

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Date:	Fri, 12 Oct 2012 23:14:28 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	"Paul E. McKenney" <paul.mckenney@...aro.org>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	alan@...rguk.ukuu.org.uk, Becky Bruce <bgillbruce@...il.com>,
	Subodh Nijsure <snijsure@...d-net.com>,
	Paul Walmsley <paul@...an.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [ 097/120] rcu: Fix day-one dyntick-idle stall-warning bug

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@...aro.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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