Re: [PATCH tip/core/rcu 1/3] rcu: Remove _rcu_barrier() dependency on __stop_machine()

Josh Triplett
2012-08-31T16:09:35Z

On Thu, Aug 30, 2012 at 12:03:01PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@linaro.org>
>
> Currently, _rcu_barrier() relies on preempt_disable() to prevent
> any CPU from going offline, which in turn depends on CPU hotplug's
> use of __stop_machine().
>
> This patch therefore makes _rcu_barrier() use get_online_cpus() to
> block CPU-hotplug operations. This has the added benefit of removing
> the need for _rcu_barrier() to adopt callbacks: Because CPU-hotplug
> operations are excluded, there can be no callbacks to adopt. This
> commit simplifies the code accordingly.
>
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Impressive simplification!

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> ---
> kernel/rcutree.c | 83 ++++++-----------------------------------------
> kernel/rcutree.h | 3 --
> kernel/rcutree_trace.c | 4 +-
> 3 files changed, 13 insertions(+), 77 deletions(-)
>
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index f280e54..9854a00 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1390,17 +1390,6 @@ static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
> int i;
> struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
>
> - /*
> - * If there is an rcu_barrier() operation in progress, then
> - * only the task doing that operation is permitted to adopt
> - * callbacks. To do otherwise breaks rcu_barrier() and friends
> - * by causing them to fail to wait for the callbacks in the
> - * orphanage.
> - */
> - if (rsp->rcu_barrier_in_progress &&
> - rsp->rcu_barrier_in_progress != current)
> - return;
> -
> /* Do the accounting first. */
> rdp->qlen_lazy += rsp->qlen_lazy;
> rdp->qlen += rsp->qlen;
> @@ -1455,9 +1444,8 @@ static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
> * The CPU has been completely removed, and some other CPU is reporting
> * this fact from process context. Do the remainder of the cleanup,
> * including orphaning the outgoing CPU's RCU callbacks, and also
> - * adopting them, if there is no _rcu_barrier() instance running.
> - * There can only be one CPU hotplug operation at a time, so no other
> - * CPU can be attempting to update rcu_cpu_kthread_task.
> + * adopting them. There can only be one CPU hotplug operation at a time,
> + * so no other CPU can be attempting to update rcu_cpu_kthread_task.
> */
> static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
> {
> @@ -1519,10 +1507,6 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
>
> #else /* #ifdef CONFIG_HOTPLUG_CPU */
>
> -static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
> -{
> -}
> -
> static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
> {
> }
> @@ -2326,13 +2310,10 @@ static void rcu_barrier_func(void *type)
> static void _rcu_barrier(struct rcu_state *rsp)
> {
> int cpu;
> - 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);
> _rcu_barrier_trace(rsp, "Begin", -1, snap);
>
> /* Take mutex to serialize concurrent rcu_barrier() requests. */
> @@ -2372,70 +2353,30 @@ static void _rcu_barrier(struct rcu_state *rsp)
> /*
> * Initialize the count to one rather than to zero in order to
> * avoid a too-soon return to zero in case of a short grace period
> - * (or preemption of this task). Also flag this task as doing
> - * an rcu_barrier(). This will prevent anyone else from adopting
> - * orphaned callbacks, which could cause otherwise failure if a
> - * CPU went offline and quickly came back online. To see this,
> - * consider the following sequence of events:
> - *
> - * 1. We cause CPU 0 to post an rcu_barrier_callback() callback.
> - * 2. CPU 1 goes offline, orphaning its callbacks.
> - * 3. CPU 0 adopts CPU 1's orphaned callbacks.
> - * 4. CPU 1 comes back online.
> - * 5. We cause CPU 1 to post an rcu_barrier_callback() callback.
> - * 6. Both rcu_barrier_callback() callbacks are invoked, awakening
> - * us -- but before CPU 1's orphaned callbacks are invoked!!!
> + * (or preemption of this task). Exclude CPU-hotplug operations
> + * to ensure that no offline CPU has callbacks queued.
> */
> init_completion(&rsp->barrier_completion);
> atomic_set(&rsp->barrier_cpu_count, 1);
> - raw_spin_lock_irqsave(&rsp->onofflock, flags);
> - rsp->rcu_barrier_in_progress = current;
> - raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> + get_online_cpus();
>
> /*
> - * Force every CPU with callbacks to register a new callback
> - * that will tell us when all the preceding callbacks have
> - * been invoked. If an offline CPU has callbacks, wait for
> - * it to either come back online or to finish orphaning those
> - * callbacks.
> + * Force each CPU with callbacks to register a new callback.
> + * When that callback is invoked, we will know that all of the
> + * corresponding CPU's preceding callbacks have been invoked.
> */
> - for_each_possible_cpu(cpu) {
> - preempt_disable();
> + for_each_online_cpu(cpu) {
> rdp = per_cpu_ptr(rsp->rda, cpu);
> - if (cpu_is_offline(cpu)) {
> - _rcu_barrier_trace(rsp, "Offline", cpu,
> - rsp->n_barrier_done);
> - preempt_enable();
> - while (cpu_is_offline(cpu) && ACCESS_ONCE(rdp->qlen))
> - schedule_timeout_interruptible(1);
> - } else if (ACCESS_ONCE(rdp->qlen)) {
> + if (ACCESS_ONCE(rdp->qlen)) {
> _rcu_barrier_trace(rsp, "OnlineQ", cpu,
> rsp->n_barrier_done);
> smp_call_function_single(cpu, rcu_barrier_func, rsp, 1);
> - preempt_enable();
> } else {
> _rcu_barrier_trace(rsp, "OnlineNQ", cpu,
> rsp->n_barrier_done);
> - preempt_enable();
> }
> }
> -
> - /*
> - * Now that all online CPUs have rcu_barrier_callback() callbacks
> - * posted, we can adopt all of the orphaned callbacks and place
> - * an rcu_barrier_callback() callback after them. When that is done,
> - * we are guaranteed to have an rcu_barrier_callback() callback
> - * following every callback that could possibly have been
> - * registered before _rcu_barrier() was called.
> - */
> - raw_spin_lock_irqsave(&rsp->onofflock, flags);
> - rcu_adopt_orphan_cbs(rsp);
> - rsp->rcu_barrier_in_progress = NULL;
> - raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> - atomic_inc(&rsp->barrier_cpu_count);
> - smp_mb__after_atomic_inc(); /* Ensure atomic_inc() before callback. */
> - rd.rsp = rsp;
> - rsp->call(&rd.barrier_head, rcu_barrier_callback);
> + put_online_cpus();
>
> /*
> * Now that we have an rcu_barrier_callback() callback on each
> @@ -2456,8 +2397,6 @@ static void _rcu_barrier(struct rcu_state *rsp)
>
> /* Other rcu_barrier() invocations can now safely proceed. */
> mutex_unlock(&rsp->barrier_mutex);
> -
> - destroy_rcu_head_on_stack(&rd.barrier_head);
> }
>
> /**
> diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> index 4d29169..94dfdf1 100644
> --- a/kernel/rcutree.h
> +++ b/kernel/rcutree.h
> @@ -398,9 +398,6 @@ struct rcu_state {
> struct rcu_head **orphan_donetail; /* Tail of above. */
> long qlen_lazy; /* Number of lazy callbacks. */
> long qlen; /* Total number of callbacks. */
> - struct task_struct *rcu_barrier_in_progress;
> - /* Task doing rcu_barrier(), */
> - /* or NULL if no barrier. */
> struct mutex barrier_mutex; /* Guards barrier fields. */
> atomic_t barrier_cpu_count; /* # CPUs waiting on. */
> struct completion barrier_completion; /* Wake at barrier end. */
> diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
> index abffb48..6a2e52a 100644
> --- a/kernel/rcutree_trace.c
> +++ b/kernel/rcutree_trace.c
> @@ -51,8 +51,8 @@ static int show_rcubarrier(struct seq_file *m, void *unused)
> struct rcu_state *rsp;
>
> for_each_rcu_flavor(rsp)
> - seq_printf(m, "%s: %c bcc: %d nbd: %lu\n",
> - rsp->name, rsp->rcu_barrier_in_progress ? 'B' : '.',
> + seq_printf(m, "%s: bcc: %d nbd: %lu\n",
> + rsp->name,
> atomic_read(&rsp->barrier_cpu_count),
> rsp->n_barrier_done);
> return 0;
> --
> 1.7.8
>