On Sun, 2012-10-14 at 16:32 -0700, Paul E. McKenney wrote:
> On Fri, Oct 12, 2012 at 11:14:28PM +0100, Ben Hutchings wrote:
> > 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?
> 
> Because the way I think of it is the way that I wrote it -- you should
> look at the value of the first pointer unless this CPU isn't up to date
> with the latest grace period, in which case you need to go one step
> farther up the array of tail pointers.

That is not the way you wrote it, since + has higher precedence than !=.

Ben.

-- 
Ben Hutchings
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