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

Oleg Nesterov
2024-05-12T16:57:04Z

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.