linux-kernel - Re: [PATCH 25/48] rcu: Mark writes to rcu_sync ->gp

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Date: Tue, 7 May 2024 10:54:41 -0400
From: Oleg Nesterov <oleg@...hat.com>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc: "Paul E . McKenney" <paulmck@...nel.org>, 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 25/48] rcu: Mark writes to rcu_sync ->gp_count field

Hello,

I feel I don't really like this patch but I am travelling without my working
laptop, can't read the source code ;) Quite possibly I am wrong, I'll return
to this when I get back on May 10.

Oleg.

On 05/07, Uladzislau Rezki (Sony) wrote:
>
> From: "Paul E. McKenney" <paulmck@...nel.org>
>
> The rcu_sync structure's ->gp_count field is updated under the protection
> of ->rss_lock, but read locklessly, and KCSAN noted the data race.
> This commit therefore uses WRITE_ONCE() to do this update to clearly
> document its racy nature.
>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  kernel/rcu/sync.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> index 86df878a2fee..6c2bd9001adc 100644
> --- a/kernel/rcu/sync.c
> +++ b/kernel/rcu/sync.c
> @@ -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) {
> @@ -151,11 +151,15 @@ 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);
> -	if (!--rsp->gp_count) {
> +	gpc = rsp->gp_count - 1;
> +	WRITE_ONCE(rsp->gp_count, gpc);
> +	if (!gpc) {
>  		if (rsp->gp_state == GP_PASSED) {
>  			WRITE_ONCE(rsp->gp_state, GP_EXIT);
>  			rcu_sync_call(rsp);
> --
> 2.39.2
>