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Date:   Mon, 21 Sep 2020 10:29:30 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>, Andrii Nakryiko <andriin@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [RFC PATCH] bpf: Fix potential call bpf_link_free() in atomic context

On Thu, Sep 17, 2020 at 12:46 AM Muchun Song <songmuchun@...edance.com> wrote:
>
> The in_atomic macro cannot always detect atomic context. In particular,
> it cannot know about held spinlocks in non-preemptible kernels. Although,
> there is no user call bpf_link_put() with holding spinlock now. Be the
> safe side, we can avoid this in the feature.
>
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---

This change seems unnecessary (or at least premature), as if we ever
get a use case that does bpf_link_put() from under held spinlock, we
should see a warning about that (and in that case I bet code can be
rewritten to not hold spinlock during bpf_link_put()). But on the
other hand it makes bpf_link_put() to follow the pattern of
bpf_map_put(), which always defers the work, so I'm ok with this. As
Song mentioned, this is not called from a performance-critical hot
path, so doesn't matter all that much.

Acked-by: Andrii Nakryiko <andriin@...com>

>  kernel/bpf/syscall.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 178c147350f5..6347be0a5c82 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -2345,12 +2345,8 @@ void bpf_link_put(struct bpf_link *link)
>         if (!atomic64_dec_and_test(&link->refcnt))
>                 return;
>
> -       if (in_atomic()) {
> -               INIT_WORK(&link->work, bpf_link_put_deferred);
> -               schedule_work(&link->work);
> -       } else {
> -               bpf_link_free(link);
> -       }
> +       INIT_WORK(&link->work, bpf_link_put_deferred);
> +       schedule_work(&link->work);
>  }
>
>  static int bpf_link_release(struct inode *inode, struct file *filp)
> --
> 2.20.1
>

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