Re: [PATCH] rcu: Make sure new krcp free business is handled after the wanted rcu grace period.

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Mar 30, 2023 at 03:31:37PM +0800, Ziwei Dai wrote:
> From: 代子为 (Ziwei Dai) <ziwei.dai@ziwei-lenovo.spreadtrum.com>
> 
> In kfree_rcu_monitor(), new free business at krcp is attached to any free
> channel at krwp. kfree_rcu_monitor() is responsible to make sure new free
> business is handled after the rcu grace period. But if there is any none-free
> channel at krwp already, that means there is an on-going rcu work,
> which will cause the kvfree_call_rcu()-triggered free business is done
> before the wanted rcu grace period ends.
> 
Right. There have to be a "Fixes:" tag.

> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8e880c0..4fe3c53 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3107,15 +3107,16 @@ static void kfree_rcu_monitor(struct work_struct *work)
>         for (i = 0; i < KFREE_N_BATCHES; i++) {
>                 struct kfree_rcu_cpu_work *krwp = &(krcp->krw_arr[i]);
> 
> -               // Try to detach bulk_head or head and attach it over any
> -               // available corresponding free channel. It can be that
> -               // a previous RCU batch is in progress, it means that
> -               // immediately to queue another one is not possible so
> -               // in that case the monitor work is rearmed.
> -               if ((!list_empty(&krcp->bulk_head[0]) && list_empty(&krwp->bulk_head_free[0])) ||
> -                       (!list_empty(&krcp->bulk_head[1]) && list_empty(&krwp->bulk_head_free[1])) ||
> -                               (READ_ONCE(krcp->head) && !krwp->head_free)) {
> -
> +               // Try to detach bulk_head or head and attach it, only when
> +               // all channels are free.  Any channel is not free means at krwp
> +               // there is on-going rcu work to handle krwp's free business.
> +               if (!list_empty(&krwp->bulk_head_free[0]) ||
> +                       !list_empty(&krwp->bulk_head_free[1]) ||
> +                               !krwp->head_free)
> +                       continue;
> +               if (!list_empty(&krcp->bulk_head[0]) ||
> +                       !list_empty(&krcp->bulk_head[1]) ||
> +                       READ_ONCE(krcp->head)) {
>                         // Channel 1 corresponds to the SLAB-pointer bulk path.
>                         // Channel 2 corresponds to vmalloc-pointer bulk path.
>                         for (j = 0; j < FREE_N_CHANNELS; j++) {
> --
> 1.9.1
> 
Give me some time to have a look at your change closer. It seems it can
trigger a rcu_work even though krcp is empty. But it is from a first
glance.

Thanks!

--
Uladzislau Rezki