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

[Josh Triplett <josh@xxxxxxxxxxxxxxxx>]

On Fri, Jun 15, 2012 at 01:13:04PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@xxxxxxxxxx>
> 
> 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.

> 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@xxxxxxxxxxxxxxxxxx>

Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>

>  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
> 
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