Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put() completes

[Andrew Morton]

(cc Ming Lei)

On Thu,  7 Apr 2022 18:33:35 +0800 Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:

> In the percpu_ref_call_confirm_rcu(), we call the wake_up_all()
> before calling percpu_ref_put(), which will cause the value of
> percpu_ref to be unstable when percpu_ref_switch_to_atomic_sync()
> returns.
> 
> 	CPU0				CPU1
> 
> percpu_ref_switch_to_atomic_sync(&ref)
> --> percpu_ref_switch_to_atomic(&ref)
>     --> percpu_ref_get(ref);	/* put after confirmation */
> 	call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu);
> 
> 					percpu_ref_switch_to_atomic_rcu
> 					--> percpu_ref_call_confirm_rcu
> 					    --> data->confirm_switch = NULL;
> 						wake_up_all(&percpu_ref_switch_waitq);
> 
>     /* here waiting to wake up */
>     wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
> 						(A)percpu_ref_put(ref);
> /* The value of &ref is unstable! */
> percpu_ref_is_zero(&ref)
> 						(B)percpu_ref_put(ref);
> 
> As shown above, assuming that the counts on each cpu add up to 0 before
> calling percpu_ref_switch_to_atomic_sync(), we expect that after switching
> to atomic mode, percpu_ref_is_zero() can return true. But actually it will
> return different values in the two cases of A and B, which is not what
> we expected.
> 
> Maybe the original purpose of percpu_ref_switch_to_atomic_sync() is
> just to ensure that the conversion to atomic mode is completed, but it
> should not return with an extra reference count.
> 
> Calling wake_up_all() after percpu_ref_put() ensures that the value of
> percpu_ref is stable after percpu_ref_switch_to_atomic_sync() returns.
> So just do it.

Thanks.  I'll grab this, but shall await input from others before doing
anything else with it.

> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> +++ b/lib/percpu-refcount.c
> @@ -154,13 +154,14 @@ static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
>  
>  	data->confirm_switch(ref);
>  	data->confirm_switch = NULL;
> -	wake_up_all(&percpu_ref_switch_waitq);
>  
>  	if (!data->allow_reinit)
>  		__percpu_ref_exit(ref);
>  
>  	/* drop ref from percpu_ref_switch_to_atomic() */
>  	percpu_ref_put(ref);
> +
> +	wake_up_all(&percpu_ref_switch_waitq);
>  }
>  
>  static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)