netdev - Re: [PATCH bpf-next v3 3/4] bpf: cgroup: properly use bpf_prog

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Date:   Tue, 28 May 2019 20:53:55 +0000
From:   Roman Gushchin <guro@...com>
To:     Stanislav Fomichev <sdf@...ichev.me>
CC:     Stanislav Fomichev <sdf@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next v3 3/4] bpf: cgroup: properly use bpf_prog_array
 api

On Tue, May 28, 2019 at 01:16:46PM -0700, Stanislav Fomichev wrote:
> On 05/28, Roman Gushchin wrote:
> > On Tue, May 28, 2019 at 11:29:45AM -0700, Stanislav Fomichev wrote:
> > > Now that we don't have __rcu markers on the bpf_prog_array helpers,
> > > let's use proper rcu_dereference_protected to obtain array pointer
> > > under mutex.
> > > 
> > > We also don't need __rcu annotations on cgroup_bpf.inactive since
> > > it's not read/updated concurrently.
> > > 
> > > v3:
> > > * amend cgroup_rcu_dereference to include percpu_ref_is_dying;
> > >   cgroup_bpf is now reference counted and we don't hold cgroup_mutex
> > >   anymore in cgroup_bpf_release
> > > 
> > > v2:
> > > * replace xchg with rcu_swap_protected
> > > 
> > > Cc: Roman Gushchin <guro@...com>
> > > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > > ---
> > >  include/linux/bpf-cgroup.h |  2 +-
> > >  kernel/bpf/cgroup.c        | 32 +++++++++++++++++++++-----------
> > >  2 files changed, 22 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
> > > index 9f100fc422c3..b631ee75762d 100644
> > > --- a/include/linux/bpf-cgroup.h
> > > +++ b/include/linux/bpf-cgroup.h
> > > @@ -72,7 +72,7 @@ struct cgroup_bpf {
> > >  	u32 flags[MAX_BPF_ATTACH_TYPE];
> > >  
> > >  	/* temp storage for effective prog array used by prog_attach/detach */
> > > -	struct bpf_prog_array __rcu *inactive;
> > > +	struct bpf_prog_array *inactive;
> > >  
> > >  	/* reference counter used to detach bpf programs after cgroup removal */
> > >  	struct percpu_ref refcnt;
> > > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > > index d995edbe816d..118b70175dd9 100644
> > > --- a/kernel/bpf/cgroup.c
> > > +++ b/kernel/bpf/cgroup.c
> > > @@ -22,6 +22,13 @@
> > >  DEFINE_STATIC_KEY_FALSE(cgroup_bpf_enabled_key);
> > >  EXPORT_SYMBOL(cgroup_bpf_enabled_key);
> > >  
> > > +#define cgroup_rcu_dereference(cgrp, p)					\
> > > +	rcu_dereference_protected(p, lockdep_is_held(&cgroup_mutex) ||	\
> > > +				  percpu_ref_is_dying(&cgrp->bpf.refcnt))
> > 
> > Some comments why percpu_ref_is_dying(&cgrp->bpf.refcnt) is enough here will
> > be appreciated.
> I was actually debating whether to just use raw
> rcu_dereference_protected(p, lockdep_is_held()) in __cgroup_bpf_query and
> rcu_dereference_protected(p, percpu_ref_is_dying()) in cgroup_bpf_release
> instead of having a cgroup_rcu_dereference which covers both cases.
> 
> Maybe that should make it more clear (and doesn't require any comment)?

Yeah, this makes total sense to me.