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SubjectRe: [PATCH tip/core/rcu 09/15] rcu: Increasing rcu_barrier() concurrency
On Fri, Jun 15, 2012 at 04:31:51PM -0700, Josh Triplett wrote:
> On Fri, Jun 15, 2012 at 02:06:04PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> >
> > The traditional rcu_barrier() implementation has serialized all requests,
> > regardless of RCU flavor, and also does not coalesce concurrent requests.
> > In the past, this has been good and sufficient.
> >
> > However, systems are getting larger and use of rcu_barrier() has been
> > increasing. This commit therefore introduces a counter-based scheme
> > that allows _rcu_barrier() calls for the same flavor of RCU to take
> > advantage of each others' work.
> >
> > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> > kernel/rcutree.c | 27 ++++++++++++++++++++++++++-
> > kernel/rcutree.h | 2 ++
> > 2 files changed, 28 insertions(+), 1 deletions(-)
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 93358d4..7c299d3 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -2291,13 +2291,32 @@ static void _rcu_barrier(struct rcu_state *rsp)
> > unsigned long flags;
> > struct rcu_data *rdp;
> > struct rcu_data rd;
> > + unsigned long snap = ACCESS_ONCE(rsp->n_barrier_done);
> > + unsigned long snap_done;
> >
> > init_rcu_head_on_stack(&rd.barrier_head);
> >
> > /* Take mutex to serialize concurrent rcu_barrier() requests. */
> > mutex_lock(&rsp->barrier_mutex);
> >
> > - smp_mb(); /* Prevent any prior operations from leaking in. */
> > + /*
> > + * Ensure tht all prior references, including to ->n_barrier_done,
> > + * are ordered before the _rcu_barrier() machinery.
> > + */
> > + smp_mb(); /* See above block comment. */
>
> If checkpatch complains about the lack of a comment to the right of a
> barrier even when the barrier has a comment directly above it, that
> seems like a bug in checkpatch that needs fixing, to prevent developers
> from having to add noise like "See above block comment.". :)

;-)

> Also: what type of barriers do mutex_lock and mutex_unlock imply? I
> assume they imply some weaker barrier than smp_mb, but I'd still assume
> they imply *some* barrier.

mutex_lock() prevents code from leaving the critical section, but is
not guaranteed to prevent code from entering the critical section.

> > + /* Recheck ->n_barrier_done to see if others did our work for us. */
> > + snap_done = ACCESS_ONCE(rsp->n_barrier_done);
> > + if (ULONG_CMP_GE(snap_done, ((snap + 1) & ~0x1) + 2)) {
>
> This calculation seems sufficiently clever that it merits an explanatory
> comment.

I will see what I can come up with.

> > + smp_mb();
> > + mutex_unlock(&rsp->barrier_mutex);
> > + return;
> > + }
> > +
> > + /* Increment ->n_barrier_done to avoid duplicate work. */
> > + ACCESS_ONCE(rsp->n_barrier_done)++;
>
> Interesting dissonance here: the use of ACCESS_ONCE with ++ implies
> exactly two accesses, rather than exactly one. What makes it safe to
> not use atomic_inc here, but not safe to drop the ACCESS_ONCE?
> Potential use of a cached value read earlier in the function?

Or, worse yet, the compiler speculating the increment and then backing
it out if the early-exit path is taken.

Thanx, Paul



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