Re: [PATCH] rcu/sync: don't read rcu_sync->gp_count lockless

[Oleg Nesterov <oleg@xxxxxxxxxx>]

On 05/12, Paul E. McKenney wrote:
>
> --- a/kernel/rcu/sync.c
> +++ b/kernel/rcu/sync.c
> @@ -151,15 +151,11 @@ void rcu_sync_enter(struct rcu_sync *rsp)
>   */
>  void rcu_sync_exit(struct rcu_sync *rsp)
>  {
> -	int gpc;
> -
>  	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
> -	WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
>
>  	spin_lock_irq(&rsp->rss_lock);
> -	gpc = rsp->gp_count - 1;
> -	WRITE_ONCE(rsp->gp_count, gpc);
> -	if (!gpc) {
> +	WARN_ON_ONCE(rsp->gp_count == 0);
> +	if (!--rsp->gp_count) {
>  		if (rsp->gp_state == GP_PASSED) {
>  			WRITE_ONCE(rsp->gp_state, GP_EXIT);
>  			rcu_sync_call(rsp);
> @@ -178,10 +174,10 @@ void rcu_sync_dtor(struct rcu_sync *rsp)
>  {
>  	int gp_state;
>
> -	WARN_ON_ONCE(READ_ONCE(rsp->gp_count));
>  	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
>
>  	spin_lock_irq(&rsp->rss_lock);
> +	WARN_ON_ONCE(rsp->gp_count);
>  	if (rsp->gp_state == GP_REPLAY)
>  		WRITE_ONCE(rsp->gp_state, GP_EXIT);
>  	gp_state = rsp->gp_state;

Thanks Paul!

But then I think this change can also revert this chunk from the previous
patch:

	@@ -122,7 +122,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
			 * we are called at early boot time but this shouldn't happen.
			 */
		}
	-	rsp->gp_count++;
	+	WRITE_ONCE(rsp->gp_count, rsp->gp_count + 1);
		spin_unlock_irq(&rsp->rss_lock);
	 
		if (gp_state == GP_IDLE) {


Thanks again,

Oleg.