netdev - Re: [PATCH v2 bpf-next 09/13] bpf: Allow reuse from waiting_for_gp

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Message-ID: <CAADnVQJViJh47Cze186XCS0_jeQMb1wu6BfVZiQL6982a_hhfg@mail.gmail.com>
Date: Sun, 25 Jun 2023 21:42:19 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Hou Tao <houtao@...weicloud.com>
Cc: Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	David Vernet <void@...ifault.com>, "Paul E. McKenney" <paulmck@...nel.org>, 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>
Subject: Re: [PATCH v2 bpf-next 09/13] bpf: Allow reuse from
 waiting_for_gp_ttrace list.

On Sun, Jun 25, 2023 at 8:30 PM Hou Tao <houtao@...weicloud.com> wrote:
>
> Hi,
>
> On 6/24/2023 11:13 AM, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@...nel.org>
> >
> > alloc_bulk() can reuse elements from free_by_rcu_ttrace.
> > Let it reuse from waiting_for_gp_ttrace as well to avoid unnecessary kmalloc().
> >
> > Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> > ---
> >  kernel/bpf/memalloc.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> > index 692a9a30c1dc..666917c16e87 100644
> > --- a/kernel/bpf/memalloc.c
> > +++ b/kernel/bpf/memalloc.c
> > @@ -203,6 +203,15 @@ static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node)
> >       if (i >= cnt)
> >               return;
> >
> > +     for (; i < cnt; i++) {
> > +             obj = llist_del_first(&c->waiting_for_gp_ttrace);
> After allowing to reuse elements from waiting_for_gp_ttrace, there may
> be concurrent llist_del_first() and llist_del_all() as shown below and
> llist_del_first() is not safe because the elements freed from free_rcu()
> could be reused immediately and head->first may be added back to
> c0->waiting_for_gp_ttrace by other process.
>
> // c0
> alloc_bulk()
>     llist_del_first(&c->waiting_for_gp_ttrace)
>
> // c1->tgt = c0
> free_rcu()
>     llist_del_all(&c->waiting_for_gp_ttrace)

I'm still thinking about how to fix the other issues you've reported,
but this one, I believe, is fine.
Are you basing 'not safe' on a comment?
Why xchg(&head->first, NULL); on one cpu and
try_cmpxchg(&head->first, &entry, next);
is unsafe?