Re: [PATCH] rcuupdate: Do a single rhp->func read in rcu_head_after_call_rcu

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Mon, Mar 11, 2019 at 05:28:03PM +0530, Neeraj Upadhyay wrote:
> Read rhp->func pointer in rcu_head_after_call_rcu() only once,
> to avoid warning in the case, where call_rcu() happens between
> two reads of rhp->func.
> 
> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>

This would more gracefully handle racing rcu_head_after_call_rcu()
with call_rcu().

But this thing is not yet used, so let's see what Neil Brown says.
If he isn't going to use it, my thought is to instead just remove
this.

							Thanx, Paul

> ---
>  include/linux/rcupdate.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 6cdb1db..174768e 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -878,9 +878,10 @@ static inline void rcu_head_init(struct rcu_head *rhp)
>  static inline bool
>  rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
>  {
> -	if (READ_ONCE(rhp->func) == f)
> +	rcu_callback_t func = READ_ONCE(rhp->func);
> +	if (func == f)
>  		return true;
> -	WARN_ON_ONCE(READ_ONCE(rhp->func) != (rcu_callback_t)~0L);
> +	WARN_ON_ONCE(func != (rcu_callback_t)~0L);
>  	return false;
>  }
> 
> -- 
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