On Mon, Aug 11, 2014 at 6:49 PM, Paul E. McKenney
<paul...@linux.vnet.ibm.com> wrote:
> From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
>
> It is expected that many sites will have CONFIG_TASKS_RCU=y, but
> will never actually invoke call_rcu_tasks(). For such sites, creating
> rcu_tasks_kthread() at boot is wasteful. This commit therefore defers
> creation of this kthread until the time of the first call_rcu_tasks().
>
> This of course means that the first call_rcu_tasks() must be invoked
> from process context after the scheduler is fully operational.
>
> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> ---
> kernel/rcu/update.c | 33 ++++++++++++++++++++++++++-------
> 1 file changed, 26 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 1256a900cd01..d997163c7e92 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -378,7 +378,12 @@ DEFINE_SRCU(tasks_rcu_exit_srcu);
> static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 10;
> module_param(rcu_task_stall_timeout, int, 0644);
>
> -/* Post an RCU-tasks callback. */
> +static void rcu_spawn_tasks_kthread(void);
> +
> +/*
> + * Post an RCU-tasks callback. First call must be from process context
> + * after the scheduler if fully operational.
> + */
> void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
> {
> unsigned long flags;
> @@ -391,8 +396,10 @@ void call_rcu_tasks(struct rcu_head *rhp, void
> (*func)(struct rcu_head *rhp))
> *rcu_tasks_cbs_tail = rhp;
> rcu_tasks_cbs_tail = &rhp->next;
> raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
> - if (needwake)
> + if (needwake) {
> + rcu_spawn_tasks_kthread();
> wake_up(&rcu_tasks_cbs_wq);
> + }
> }
> EXPORT_SYMBOL_GPL(call_rcu_tasks);
>
> @@ -618,15 +625,27 @@ static int __noreturn rcu_tasks_kthread(void *arg)
> }
> }
>
> -/* Spawn rcu_tasks_kthread() at boot time. */
> -static int __init rcu_spawn_tasks_kthread(void)
> +/* Spawn rcu_tasks_kthread() at first call to call_rcu_tasks(). */
> +static void rcu_spawn_tasks_kthread(void)
> {
> - struct task_struct __maybe_unused *t;
> + static DEFINE_MUTEX(rcu_tasks_kthread_mutex);
> + static struct task_struct *rcu_tasks_kthread_ptr;
> + struct task_struct *t;
>
> + if (ACCESS_ONCE(rcu_tasks_kthread_ptr)) {
> + smp_mb(); /* Ensure caller sees full kthread. */
> + return;
> + }
I don't see the need for this smp_mb(). The caller has already seen
that rcu_tasks_kthread_ptr is assigned. What are we ensuring with this
barrier again?
an smp_rmb() before this ACCESS_ONCE() and an smp_wmb() after
assigning to rcu_tasks_kthread_ptr should be enough, right?
> + mutex_lock(&rcu_tasks_kthread_mutex);
> + if (rcu_tasks_kthread_ptr) {
> + mutex_unlock(&rcu_tasks_kthread_mutex);
> + return;
> + }
> t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread");
> BUG_ON(IS_ERR(t));
> - return 0;
> + smp_mb(); /* Ensure others see full kthread. */
> + ACCESS_ONCE(rcu_tasks_kthread_ptr) = t;
Isn't it better to reverse these two statements and change as follows?
ACCESS_ONCE(rcu_tasks_kthread_ptr) = t;
smp_wmb();
or
smp_store_release(rcu_tasks_kthread_ptr, t);
will ensure that this write to rcu_task_kthread_ptr is ordered with
the previous read. I recently read memory-barriers.txt, so please
excuse me if I am totally wrong. But I am confused! :(
> + mutex_unlock(&rcu_tasks_kthread_mutex);
> }
> -early_initcall(rcu_spawn_tasks_kthread);
>
> #endif /* #ifdef CONFIG_TASKS_RCU */
> --
> 1.8.1.5
>
--
Pranith
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