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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>,
	Daniel Borkmann <daniel@iogearbox.net>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH v4 bpf-next 1/5] bpf: Remove redundant synchronize_rcu.
Date: Mon, 29 Jun 2020 19:56:13 -0700	[thread overview]
Message-ID: <20200630025613.scvhmqootlnxp7sx@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <CAEf4BzZ4oEbONjbW5D5rngeiuT-BzREMKBz9H_=gzfdvBbvMOQ@mail.gmail.com>

On Mon, Jun 29, 2020 at 06:08:48PM -0700, Andrii Nakryiko wrote:
> On Mon, Jun 29, 2020 at 5:58 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Mon, Jun 29, 2020 at 5:35 PM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
> > > From: Alexei Starovoitov <ast@kernel.org>
> > >
> > > bpf_free_used_maps() or close(map_fd) will trigger map_free callback.
> > > bpf_free_used_maps() is called after bpf prog is no longer executing:
> > > bpf_prog_put->call_rcu->bpf_prog_free->bpf_free_used_maps.
> > > Hence there is no need to call synchronize_rcu() to protect map elements.
> > >
> > > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > > ---
> >
> > Seems correct. And nice that maps don't have to care about this anymore.
> >
> 
> Actually, what about the map-in-map case?
> 
> What if you had an array-of-maps with an inner map element. It is the
> last reference to that map. Now you have two BPF prog executions in
> parallel. One looked up that inner map and is updating it at the
> moment. Another execution at the same time deletes that map. That
> deletion will call bpf_map_put(), which without synchronize_rcu() will
> free memory. All the while the former BPF program execution is still
> working with that map.

The delete of that inner map can only be done via sys_bpf() and there
we do maybe_wait_bpf_programs() exactly to avoid this kind of problems.
It's also necessary for user space. When the user is doing map_update/delete
of inner map as soon as syscall returns the user can process
old map with guarantees that no bpf prog is touching inner map.

  reply	other threads:[~2020-06-30  2:56 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-30  0:34 [PATCH v4 bpf-next 0/5] bpf: Introduce minimal support for sleepable progs Alexei Starovoitov
2020-06-30  0:34 ` [PATCH v4 bpf-next 1/5] bpf: Remove redundant synchronize_rcu Alexei Starovoitov
2020-06-30  0:58   ` Andrii Nakryiko
2020-06-30  1:08     ` Andrii Nakryiko
2020-06-30  2:56       ` Alexei Starovoitov [this message]
2020-06-30  3:31         ` Andrii Nakryiko
2020-06-30  0:34 ` [PATCH v4 bpf-next 2/5] bpf: Introduce sleepable BPF programs Alexei Starovoitov
2020-06-30  1:15   ` Andrii Nakryiko
2020-06-30  3:03     ` Alexei Starovoitov
2020-06-30  0:34 ` [PATCH v4 bpf-next 3/5] bpf: Add bpf_copy_from_user() helper Alexei Starovoitov
2020-06-30  1:18   ` Andrii Nakryiko
2020-06-30  0:34 ` [PATCH v4 bpf-next 4/5] libbpf: support sleepable progs Alexei Starovoitov
2020-06-30  0:34 ` [PATCH v4 bpf-next 5/5] selftests/bpf: Add sleepable tests Alexei Starovoitov
2020-06-30  1:25   ` Andrii Nakryiko
2020-06-30  3:06     ` Alexei Starovoitov
2020-06-30  3:34       ` Andrii Nakryiko

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