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

[Oleg Nesterov]

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.