linux-kernel - Re: [PATCH] rcu/sync: don't read rcu_sync->gp

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Date: Sun, 12 May 2024 18:55:29 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>, RCU <rcu@...r.kernel.org>,
	Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Boqun Feng <boqun.feng@...il.com>, Hillf Danton <hdanton@...a.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] rcu/sync: don't read rcu_sync->gp_count lockless

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.