From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: syzbot <syzbot+1fa663a2100308ab6eab@syzkaller.appspotmail.com>,
Andrii Nakryiko <andrii@kernel.org>,
Alexei Starovoitov <ast@kernel.org>, bpf <bpf@vger.kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Eddy Z <eddyz87@gmail.com>, Hao Luo <haoluo@google.com>,
John Fastabend <john.fastabend@gmail.com>,
Jiri Olsa <jolsa@kernel.org>, KP Singh <kpsingh@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Martin KaFai Lau <martin.lau@linux.dev>,
Stanislav Fomichev <sdf@google.com>, Song Liu <song@kernel.org>,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
Yonghong Song <yonghong.song@linux.dev>,
Uladzislau Rezki <urezki@gmail.com>,
rcu@vger.kernel.org
Subject: Re: false positive deadlock? Was: [syzbot] [bpf?] possible deadlock in kvfree_call_rcu
Date: Wed, 27 Mar 2024 17:12:57 -0700 [thread overview]
Message-ID: <CAADnVQLKzumBG0GG3cSxQMVRKM_Km9YMb6ub=n9C957qx8=ExQ@mail.gmail.com> (raw)
In-Reply-To: <4dc396d9-b57c-4ccf-bbef-887d5a42ab66@paulmck-laptop>
On Tue, Mar 26, 2024 at 10:04 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Tue, Mar 26, 2024 at 09:37:43PM -0700, Paul E. McKenney wrote:
> > On Tue, Mar 26, 2024 at 12:53:35PM -0700, Alexei Starovoitov wrote:
> > > Hi Paul,
> > >
> > > syzbot found an interesting false positive deadlock.
> > > See below.
> > > My understanding is the following:
> > >
> > > cpu 2:
> > > grabs timer_base lock
> > > spins on bpf_lpm lock
> > >
> > > cpu 1:
> > > grab rcu krcp lock
> > > spins on timer_base lock
> > >
> > > cpu 0:
> > > grab bpf_lpm lock
> > > spins on rcu krcp lock
> > >
> > > bpf_lpm lock can be the same.
> > > timer_base lock can also be the same due to timer migration.
> > >
> > > but rcu krcp lock is always per-cpu, so it cannot be the same lock.
> > > Hence it's a false positive, but still interesting.
> > >
> > > I don't think rcu can tell lockdep that these are different locks.
> >
> > It might be possible. I will play with this tomorrow, modeling after
> > the use of lockdep_set_class_and_name() in rcu_init_one(). I am a bit
> > concerned about systems with thousands of CPUs, but it just might be OK.
>
> Except that each of the resulting separate locks would eventually be
> classified as participating in the same type of potential deadlock cycle. :-(
That sounds like we have to address it on bpf side,
since we're being spammed with syzbot reports of various
forms and all of them are about this false positive.
next prev parent reply other threads:[~2024-03-28 0:13 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <000000000000f2f0c0061494e610@google.com>
[not found] ` <CAADnVQKEU0yL=GpaNQw=z0J9f_S=i+rQp9QZK3mYv6632WhfUg@mail.gmail.com>
2024-03-27 4:37 ` false positive deadlock? Was: [syzbot] [bpf?] possible deadlock in kvfree_call_rcu Paul E. McKenney
2024-03-27 5:04 ` Paul E. McKenney
2024-03-28 0:12 ` Alexei Starovoitov [this message]
2024-03-27 23:27 ` syzbot
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