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From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: "Björn Töpel" <bjorn.topel@gmail.com>,
	ast@kernel.org, daniel@iogearbox.net, netdev@vger.kernel.org,
	bpf@vger.kernel.org
Cc: "Björn Töpel" <bjorn.topel@intel.com>,
	magnus.karlsson@intel.com, maciej.fijalkowski@intel.com,
	kuba@kernel.org, jonathan.lemon@gmail.com, maximmi@nvidia.com,
	davem@davemloft.net, hawk@kernel.org, john.fastabend@gmail.com,
	ciara.loftus@intel.com, weqaar.a.janjua@intel.com
Subject: Re: [PATCH bpf-next v2 1/8] xdp: restructure redirect actions
Date: Wed, 20 Jan 2021 13:44:24 +0100	[thread overview]
Message-ID: <87bldjeq1j.fsf@toke.dk> (raw)
In-Reply-To: <20210119155013.154808-2-bjorn.topel@gmail.com>

Björn Töpel <bjorn.topel@gmail.com> writes:

> From: Björn Töpel <bjorn.topel@intel.com>
>
> The XDP_REDIRECT implementations for maps and non-maps are fairly
> similar, but obviously need to take different code paths depending on
> if the target is using a map or not. Today, the redirect targets for
> XDP either uses a map, or is based on ifindex.
>
> Future commits will introduce yet another redirect target via the a
> new helper, bpf_redirect_xsk(). To pave the way for that, we introduce
> an explicit redirect type to bpf_redirect_info. This makes the code
> easier to follow, and makes it easier to add new redirect targets.
>
> Further, using an explicit type in bpf_redirect_info has a slight
> positive performance impact by avoiding a pointer indirection for the
> map type lookup, and instead use the hot cacheline for
> bpf_redirect_info.
>
> The bpf_redirect_info flags member is not used by XDP, and not
> read/written any more. The map member is only written to when
> required/used, and not unconditionally.

I like the simplification. However, the handling of map clearing becomes
a bit murky with this change:

You're not changing anything in bpf_clear_redirect_map(), and you're
removing most of the reads and writes of ri->map. Instead,
bpf_xdp_redirect_map() will store the bpf_dtab_netdev pointer in
ri->tgt_value, which xdp_do_redirect() will just read and use without
checking. But if the map element (or the entire map) has been freed in
the meantime that will be a dangling pointer. I *think* the RCU callback
in dev_map_delete_elem() and the rcu_barrier() in dev_map_free()
protects against this, but that is by no means obvious. So confirming
this, and explaining it in a comment would be good.

Also, as far as I can tell after this, ri->map is only used for the
tracepoint. So how about just storing the map ID and getting rid of the
READ/WRITE_ONCE() entirely?

(Oh, and related to this I think this patch set will conflict with
Hangbin's multi-redirect series, so maybe you two ought to coordinate? :))

-Toke


  reply	other threads:[~2021-01-20 20:12 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-19 15:50 [PATCH bpf-next v2 0/8] Introduce bpf_redirect_xsk() helper Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 1/8] xdp: restructure redirect actions Björn Töpel
2021-01-20 12:44   ` Toke Høiland-Jørgensen [this message]
2021-01-20 13:40     ` Björn Töpel
2021-01-20 14:52       ` Toke Høiland-Jørgensen
2021-01-20 15:49         ` Björn Töpel
2021-01-20 16:30           ` Toke Høiland-Jørgensen
2021-01-20 17:26             ` Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 2/8] xsk: remove explicit_free parameter from __xsk_rcv() Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 3/8] xsk: fold xp_assign_dev and __xp_assign_dev Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 4/8] xsk: register XDP sockets at bind(), and add new AF_XDP BPF helper Björn Töpel
2021-01-20  8:25   ` kernel test robot
2021-01-20  8:41     ` Björn Töpel
2021-01-20  8:50   ` kernel test robot
2021-01-20 12:50   ` Toke Høiland-Jørgensen
2021-01-20 13:25     ` Björn Töpel
2021-01-20 14:54       ` Toke Høiland-Jørgensen
2021-01-20 15:18         ` Björn Töpel
2021-01-20 17:29           ` Toke Høiland-Jørgensen
2021-01-20 18:22             ` Björn Töpel
2021-01-20 20:26               ` Toke Høiland-Jørgensen
2021-01-20 21:15                 ` Alexei Starovoitov
2021-01-21  8:18                   ` Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 5/8] libbpf, xsk: select AF_XDP BPF program based on kernel version Björn Töpel
2021-01-20 12:52   ` Toke Høiland-Jørgensen
2021-01-20 13:25     ` Björn Töpel
2021-01-20 14:49       ` Björn Töpel
2021-01-20 15:11         ` Toke Høiland-Jørgensen
2021-01-20 15:27           ` Björn Töpel
2021-01-20 17:30             ` Toke Høiland-Jørgensen
2021-01-20 18:25             ` Alexei Starovoitov
2021-01-20 18:30               ` Björn Töpel
2021-01-20 14:56       ` Toke Høiland-Jørgensen
2021-01-19 15:50 ` [PATCH bpf-next v2 6/8] libbpf, xsk: select bpf_redirect_xsk(), if supported Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 7/8] selftest/bpf: add XDP socket tests for bpf_redirect_{xsk, map}() Björn Töpel
2021-01-21  7:39   ` Andrii Nakryiko
2021-01-21 12:31     ` Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 8/8] selftest/bpf: remove a lot of ifobject casting in xdpxceiver Björn Töpel
2021-01-20 13:15 ` [PATCH bpf-next v2 0/8] Introduce bpf_redirect_xsk() helper Maxim Mikityanskiy
2021-01-20 13:27   ` Björn Töpel
2021-01-20 15:57   ` Jesper Dangaard Brouer
2021-01-20 16:19     ` Maciej Fijalkowski
2021-01-21 17:01       ` Jesper Dangaard Brouer
2021-01-22  8:59         ` Magnus Karlsson
2021-01-22  9:45           ` Maciej Fijalkowski

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