netdev - Re: [PATCH v4 bpf-next 09/14] bpf: Allow reuse from waiting_for_gp

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Date: Thu, 6 Jul 2023 19:12:21 -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 v4 bpf-next 09/14] bpf: Allow reuse from
 waiting_for_gp_ttrace list.

On Thu, Jul 6, 2023 at 7:07 PM Hou Tao <houtao@...weicloud.com> wrote:
>
> Hi,
>
> On 7/6/2023 11:34 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 | 16 ++++++++++------
> >  1 file changed, 10 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> > index 9986c6b7df4d..e5a87f6cf2cc 100644
> > --- a/kernel/bpf/memalloc.c
> > +++ b/kernel/bpf/memalloc.c
> > @@ -212,6 +212,15 @@ 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);
> > +             if (!obj)
> > +                     break;
> > +             add_obj_to_free_list(c, obj);
> > +     }
> > +     if (i >= cnt)
> > +             return;
>
> I still think using llist_del_first() here is not safe as reported in
> [1]. Not sure whether or not invoking enque_to_free() firstly for
> free_llist_extra will close the race completely. Will check later.

lol. see my reply a second ago in the other thread.

and it's not just waiting_for_gp_ttrace. free_by_rcu_ttrace is similar.