netdev - Re: [PATCH bpf] bpf: Use hlist_add_head_rcu when linking to sk

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Message-ID: <20200917180100.3tm4dcafwqtjtl6n@kafai-mbp>
Date:   Thu, 17 Sep 2020 11:01:00 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: Use hlist_add_head_rcu when linking to
 sk_storage

On Thu, Sep 17, 2020 at 10:47:39AM -0700, Alexei Starovoitov wrote:
> On Wed, Sep 16, 2020 at 01:09:25PM -0700, Martin KaFai Lau wrote:
> > The sk_storage->list will be traversed by rcu reader in parallel.
> > Thus, hlist_add_head_rcu() is needed in __selem_link_sk().  This
> > patch fixes it.
> > 
> > This part of the code has recently been refactored in bpf-next.
> > A separate fix will be provided for the bpf-next tree.
> > 
> > Fixes: 6ac99e8f23d4 ("bpf: Introduce bpf sk local storage")
> > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > ---
> >  net/core/bpf_sk_storage.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
> > index b988f48153a4..d4d2a56e9d4a 100644
> > --- a/net/core/bpf_sk_storage.c
> > +++ b/net/core/bpf_sk_storage.c
> > @@ -219,7 +219,7 @@ static void __selem_link_sk(struct bpf_sk_storage *sk_storage,
> >  			    struct bpf_sk_storage_elem *selem)
> >  {
> >  	RCU_INIT_POINTER(selem->sk_storage, sk_storage);
> > -	hlist_add_head(&selem->snode, &sk_storage->list);
> > +	hlist_add_head_rcu(&selem->snode, &sk_storage->list);
> >  }
> 
> Applying the same, yet very different from git point of view, patch to
> bpf and bpf-next trees will create a ton of confusion for everyone.
> I prefer to take this fix (in bpf-next form) into bpf-next only and apply
> this fix (in bpf form) to 5.9 and stable after the merge window.
> The code has been around since April 2019 and it wasn't hit in prod,
> so I don't think there is urgency.
> Agree?
Yep, agree.