Re: [PATCH tip/core/rcu 1/3] rcu-tasks: *_ONCE() for rcu_tasks_cbs_head

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Feb 14, 2020 at 04:25:18PM -0800, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> The RCU tasks list of callbacks, rcu_tasks_cbs_head, is sampled locklessly
> by rcu_tasks_kthread() when waiting for work to do.  This commit therefore
> applies READ_ONCE() to that lockless sampling and WRITE_ONCE() to the
> single potential store outside of rcu_tasks_kthread.
> 
> This data race was reported by KCSAN.  Not appropriate for backporting
> due to failure being unlikely.

What failure is possible here? AFAICT this is (again) one of them
load-complare-against-constant-discard patterns that are impossible to
mess up.

> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>  kernel/rcu/update.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 6c4b862..a27df76 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -528,7 +528,7 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
>  	rhp->func = func;
>  	raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
>  	needwake = !rcu_tasks_cbs_head;
> -	*rcu_tasks_cbs_tail = rhp;
> +	WRITE_ONCE(*rcu_tasks_cbs_tail, rhp);
>  	rcu_tasks_cbs_tail = &rhp->next;
>  	raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
>  	/* We can't create the thread unless interrupts are enabled. */
> @@ -658,7 +658,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
>  		/* If there were none, wait a bit and start over. */
>  		if (!list) {
>  			wait_event_interruptible(rcu_tasks_cbs_wq,
> -						 rcu_tasks_cbs_head);
> +						 READ_ONCE(rcu_tasks_cbs_head));
>  			if (!rcu_tasks_cbs_head) {
>  				WARN_ON(signal_pending(current));
>  				schedule_timeout_interruptible(HZ/10);
> -- 
> 2.9.5
>