netdev - Re: [PATCH v3 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu() similar to kfree

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Date: Wed, 5 Jul 2023 20:25:22 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Hou Tao <houtao@...weicloud.com>
Cc: Tejun Heo <tj@...nel.org>, rcu@...r.kernel.org, 
	Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Kernel Team <kernel-team@...com>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, David Vernet <void@...ifault.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v3 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu()
 similar to kfree_rcu().

On Wed, Jun 28, 2023 at 7:24 PM Hou Tao <houtao@...weicloud.com> wrote:
>
> I think the race could be fixed by checking c->draining in
> do_call_rcu_ttrace() when atomic_xchg() returns 1 as shown below:
>
> diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> index 2bdb894392c5..9f41025560bd 100644
> --- a/kernel/bpf/memalloc.c
> +++ b/kernel/bpf/memalloc.c
> @@ -303,8 +303,13 @@ static void do_call_rcu_ttrace(struct bpf_mem_cache *c)
>  {
>         struct llist_node *llnode, *t;
>
> -       if (atomic_xchg(&c->call_rcu_ttrace_in_progress, 1))
> +       if (atomic_xchg(&c->call_rcu_ttrace_in_progress, 1)) {
> +               if (READ_ONCE(c->draining)) {
> +                       llnode = llist_del_all(&c->free_by_rcu_ttrace);
> +                       free_all(llnode, !!c->percpu_size);
> +               }
>                 return;
> +       }

I managed to repro with your extra check-leaks patch that I will
include in the series.
The fix also makes sense.
Thanks