Re: [PATCH tip/core/rcu 03/14] rcu: Add ACCESS_ONCE() to ->qlen accesses

Paul E. McKenney
2012-06-15T22:24:52Z

On Fri, Jun 15, 2012 at 01:45:00PM -0700, Josh Triplett wrote:
> On Fri, Jun 15, 2012 at 01:13:04PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> >
> > The _rcu_barrier() function accesses other CPUs' rcu_data structure's
> > ->qlen field without benefit of locking. This commit therefore adds
> > the required ACCESS_ONCE() wrappers around accesses and updates that
> > need it.
>
> This type of restriction makes me wonder if we could add some kind of
> attribute to fields like qlen to make GCC or sparse help enforce this.

It is worth thinking about. Should spark some spirited discussions. ;-)

> > ACCESS_ONCE() is not needed when a CPU accesses its own ->qlen, or
> > in code that cannot run while _rcu_barrier() is sampling ->qlen fields.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Thank you!

Thanx, Paul

> > kernel/rcutree.c | 8 ++++----
> > 1 files changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index d938671..cdc101e 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1349,7 +1349,7 @@ rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
> > rsp->qlen += rdp->qlen;
> > rdp->n_cbs_orphaned += rdp->qlen;
> > rdp->qlen_lazy = 0;
> > - rdp->qlen = 0;
> > + ACCESS_ONCE(rdp->qlen) = 0;
> > }
> >
> > /*
> > @@ -1597,7 +1597,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
> > }
> > smp_mb(); /* List handling before counting for rcu_barrier(). */
> > rdp->qlen_lazy -= count_lazy;
> > - rdp->qlen -= count;
> > + ACCESS_ONCE(rdp->qlen) -= count;
> > rdp->n_cbs_invoked += count;
> >
> > /* Reinstate batch limit if we have worked down the excess. */
> > @@ -1886,7 +1886,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
> > rdp = this_cpu_ptr(rsp->rda);
> >
> > /* Add the callback to our list. */
> > - rdp->qlen++;
> > + ACCESS_ONCE(rdp->qlen)++;
> > if (lazy)
> > rdp->qlen_lazy++;
> > else
> > @@ -2420,7 +2420,7 @@ rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
> > rdp->grpmask = 1UL << (cpu - rdp->mynode->grplo);
> > init_callback_list(rdp);
> > rdp->qlen_lazy = 0;
> > - rdp->qlen = 0;
> > + ACCESS_ONCE(rdp->qlen) = 0;
> > rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
> > WARN_ON_ONCE(rdp->dynticks->dynticks_nesting != DYNTICK_TASK_EXIT_IDLE);
> > WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1);
> > --
> > 1.7.8
> >
>