Re: [PATCH bpf-next 1/2] bpf: Allow bpf_local_storage to be used by sleepable programs

From: Martin KaFai Lau <kafai@fb.com>
To: KP Singh <kpsingh@kernel.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
	bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>, Jann Horn <jannh@google.com>,
	Florent Revest <revest@chromium.org>,
	Brendan Jackman <jackmanb@chromium.org>,
	Yonghong Song <yhs@fb.com>
Subject: Re: [PATCH bpf-next 1/2] bpf: Allow bpf_local_storage to be used by sleepable programs
Date: Tue, 23 Nov 2021 16:18:18 -0800	[thread overview]
Message-ID: <20211124001818.43fl5b7npaiaoj7k@kafai-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <CACYkzJ6TP19iV3hstamRge42R-7uKynbMQKcMHVLzCyTVEzVKw@mail.gmail.com>

On Wed, Nov 24, 2021 at 12:14:29AM +0100, KP Singh wrote:
> On Tue, Nov 23, 2021 at 11:30 PM Martin KaFai Lau <kafai@fb.com> wrote:
> >
> > On Tue, Nov 23, 2021 at 10:22:04AM -0800, Paul E. McKenney wrote:
> > > On Tue, Nov 23, 2021 at 06:11:14PM +0100, KP Singh wrote:
> > > > On Thu, Sep 2, 2021 at 6:45 AM Martin KaFai Lau <kafai@fb.com> wrote:
> > > > > I think the global lock will be an issue for the current non-sleepable
> > > > > netdev bpf-prog which could be triggered by external traffic,  so a flag
> > > > > is needed here to provide a fast path.  I suspect other non-prealloc map
> > > > > may need it in the future, so probably
> > > > > s/BPF_F_SLEEPABLE_STORAGE/BPF_F_SLEEPABLE/ instead.
> > > >
> > > > I was re-working the patches and had a couple of questions.
> > > >
> > > > There are two data structures that get freed under RCU here:
> > > >
> > > > struct bpf_local_storage
> > > > struct bpf_local_storage_selem
> > > >
> > > > We can choose to free the bpf_local_storage_selem under
> > > > call_rcu_tasks_trace based on
> > > > whether the map it belongs to is sleepable with something like:
> > > >
> > > > if (selem->sdata.smap->map.map_flags & BPF_F_SLEEPABLE_STORAGE)
> > Paul's current work (mentioned by his previous email) will improve the
> > performance of call_rcu_tasks_trace, so it probably can avoid the
> > new BPF_F_SLEEPABLE flag and make it easier to use.
> >
> > > >     call_rcu_tasks_trace(&selem->rcu, bpf_selem_free_rcu);
> > > > else
> > > >     kfree_rcu(selem, rcu);
> > > >
> > > > Questions:
> > > >
> > > > * Can we free bpf_local_storage under kfree_rcu by ensuring it's
> > > >   always accessed in a  classical RCU critical section?
> > >>    Or maybe I am missing something and this also needs to be freed
> > > >   under trace RCU if any of the selems are from a sleepable map.
> > In the inode_storage_lookup() of this patch:
> >
> > +#define bpf_local_storage_rcu_lock_held()                      \
> > +       (rcu_read_lock_held() || rcu_read_lock_trace_held() ||  \
> > +        rcu_read_lock_bh_held())
> >
> > @@ -44,7 +45,8 @@ static struct bpf_local_storage_data *inode_storage_lookup(struct inode *inode,
> >         if (!bsb)
> >                 return NULL;
> >
> > -       inode_storage = rcu_dereference(bsb->storage);
> > +       inode_storage = rcu_dereference_protected(bsb->storage,
> > +                                                 bpf_local_storage_rcu_lock_held());
> >
> > Thus, it is not always in classical RCU critical.
> 
> I was planning on adding a classical RCU read side critical section
> whenever we called the lookup functions.
> 
> Would that have worked? (for the sake of learning).
ah. ic. You meant local_storage could be under rcu_read_lock()
if we wanted to since it is not exposed to the sleepable
bpf_prog which is under rcu_read_lock_trace()?

It should work after a quick thought but then we
need to figure out the needed places to add
an extra rcu_read_lock().  What is the
reason for doing this?