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Message-ID: <20190528201646.GE3032@mini-arch> Date: Tue, 28 May 2019 13:16:46 -0700 From: Stanislav Fomichev <sdf@...ichev.me> To: Roman Gushchin <guro@...com> 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 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)?
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