[PATCH] rcu: Remove unnecessary READ_ONCE in rcu_preempt_read_enter/exit

[PATCH] rcu: Remove unnecessary READ_ONCE in rcu_preempt_read_enter/exit

[Alan Huang]

The two READ_ONCEs here imply that there might be updater concurrently
modifies rcu_read_lock_nesting, although it's true there could be
interrupt handlers enter rcu RSCS, but rcu_read_lock_nesting should
remain unchanged after they finished. That is, READ_ONCE and then
WRITE_ONCE the same variable is not a valid pattern, this patch thus
removes the two READ_ONCEs.

Signed-off-by: Alan Huang <mmpgouride@gmail.com>
---
 kernel/rcu/tree_plugin.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 340bbefe5f65..c90edbb7fa40 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -376,12 +376,12 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
 
 static void rcu_preempt_read_enter(void)
 {
-	WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1);
+	WRITE_ONCE(current->rcu_read_lock_nesting, current->rcu_read_lock_nesting + 1);
 }
 
 static int rcu_preempt_read_exit(void)
 {
-	int ret = READ_ONCE(current->rcu_read_lock_nesting) - 1;
+	int ret = current->rcu_read_lock_nesting - 1;
 
 	WRITE_ONCE(current->rcu_read_lock_nesting, ret);
 	return ret;
-- 
2.34.1

Re: [PATCH] rcu: Remove unnecessary READ_ONCE in rcu_preempt_read_enter/exit

[Paul E. McKenney]

On Sun, Mar 31, 2024 at 09:19:32AM +0000, Alan Huang wrote:
> The two READ_ONCEs here imply that there might be updater concurrently
> modifies rcu_read_lock_nesting, although it's true there could be
> interrupt handlers enter rcu RSCS, but rcu_read_lock_nesting should
> remain unchanged after they finished. That is, READ_ONCE and then
> WRITE_ONCE the same variable is not a valid pattern, this patch thus
> removes the two READ_ONCEs.

This is true, but I will take the READ_ONCE() calls over the KCSAN
complaints.  So I must avoid taking this one.

Please note that RCU uses strict KCSAN configuration [1].

							Thanx, Paul

[1] https://docs.google.com/document/d/1FwZaXSg3A55ivVoWffA9iMuhJ3_Gmj_E494dLYjjyLQ/edit?usp=sharing

> Signed-off-by: Alan Huang <mmpgouride@gmail.com>
> ---
>  kernel/rcu/tree_plugin.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 340bbefe5f65..c90edbb7fa40 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -376,12 +376,12 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
>  
>  static void rcu_preempt_read_enter(void)
>  {
> -	WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1);
> +	WRITE_ONCE(current->rcu_read_lock_nesting, current->rcu_read_lock_nesting + 1);
>  }
>  
>  static int rcu_preempt_read_exit(void)
>  {
> -	int ret = READ_ONCE(current->rcu_read_lock_nesting) - 1;
> +	int ret = current->rcu_read_lock_nesting - 1;
>  
>  	WRITE_ONCE(current->rcu_read_lock_nesting, ret);
>  	return ret;
> -- 
> 2.34.1
> 
>