Messages in this thread | | | Date | Fri, 15 Jun 2012 16:31:51 -0700 | From | Josh Triplett <> | Subject | Re: [PATCH tip/core/rcu 09/15] rcu: Increasing rcu_barrier() concurrency |
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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.
> + /* 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.
> + 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?
- Josh Triplett
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