Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data access

Sebastian Andrzej Siewior
2020-05-20T18:28:40Z

On 2020-05-20 13:42:59 [-0400], Joel Fernandes wrote:
> Hi Sebastian,
Hi Joel,

> For pointer stability, can we just use get_local_ptr() and put_local_ptr()
> instead of adding an extra lock? This keeps the pointer stable while keeping
> the section preemptible on -rt. And we already have a lock in rcu_data, I
> prefer not to add another lock if possible.

What is this get_local_ptr() doing? I can't find it anywhere…

> I wrote a diff below with get_local_ptr() (just build tested). Does this
> solve your issue?

see below.

> > I remember Paul looked at that patch a few years ago and he said that
> > that disabling interrupts here is important and matches the other part
> > instance where the interrupts are disabled. Looking at it now, it seems
> > that there is just pointer stability but I can't tell if
> > rcu_segcblist_pend_cbs() needs more than just this.
>
> Which 'other part' are you referring to? Your patch removed local_irq_save()
> from other places as well right?

The patch converted hunks.

> thanks,
>
> - Joel
>
> ---8<-----------------------
>
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 8ff71e5d0fe8b..5f49919205317 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -778,13 +778,17 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
> unsigned long tlast;
>
> /* If the local srcu_data structure has callbacks, not idle. */
> - local_irq_save(flags);
> - sdp = this_cpu_ptr(ssp->sda);
> + sdp = get_local_ptr(ssp->sda);
> + spin_lock_irqsave_rcu_node(sdp, flags);

You acquire the node lock which was not acquired before. Is that okay?
How is get_local_ptr() different to raw_cpu_ptr()?

> if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
> - local_irq_restore(flags);
> + spin_unlock_irqrestore_rcu_node(sdp, flags);
> + put_local_ptr(sdp);
> return false; /* Callbacks already present, so not idle. */
> }
> - local_irq_restore(flags);
> +
> + spin_unlock_irqrestore_rcu_node(sdp, flags);
> + put_local_ptr(sdp);
>
> /*
> * No local callbacks, so probabalistically probe global state.

Sebastian