linux-kernel - Re: [PATCH] bpf: force inc_active()/dec

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Date:   Tue, 25 Jul 2023 11:15:53 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Yafang Shao <laoar.shao@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Hou Tao <houtao1@...wei.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: force inc_active()/dec_active() to be inline functions

On Mon, Jul 24, 2023 at 1:41 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Mon, Jul 24, 2023, at 21:15, Alexei Starovoitov wrote:
> > On Mon, Jul 24, 2023 at 11:30 AM Arnd Bergmann <arnd@...nel.org> wrote:
> >> On Mon, Jul 24, 2023, at 20:13, Arnd Bergmann wrote:
> >>
> >> I have a minimized test case at https://godbolt.org/z/hK4ev17fv
> >> that shows the problem happening with all versions of gcc
> >> (4.1 through 14.0) if I force the dec_active() function to be
> >> inline and force inc_active() to be non-inline.
> >
> > That's a bit of cheating, but I see your point now.
> > How about we do:
> > diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> > index 51d6389e5152..3fa0944cb975 100644
> > --- a/kernel/bpf/memalloc.c
> > +++ b/kernel/bpf/memalloc.c
> > @@ -183,11 +183,11 @@ static void inc_active(struct bpf_mem_cache *c,
> > unsigned long *flags)
> >         WARN_ON_ONCE(local_inc_return(&c->active) != 1);
> >  }
> >
> > -static void dec_active(struct bpf_mem_cache *c, unsigned long flags)
> > +static void dec_active(struct bpf_mem_cache *c, unsigned long *flags)
> >  {
> >         local_dec(&c->active);
> >         if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > -               local_irq_restore(flags);
> > +               local_irq_restore(*flags);
> >  }
>
>
> Sure, that's fine. Between this and the two suggestions I had
> (__always_inline or passing the flags from  inc_active as a
> return code), I don't have a strong preference, so pick whichever
> you like.

I think:
static void dec_active(struct bpf_mem_cache *c, unsigned long *flags)
is cleaner.
Could you send a patch?