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From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: "Björn Töpel" <bjorn.topel@intel.com>,
	"Björn Töpel" <bjorn.topel@gmail.com>,
	ast@kernel.org, daniel@iogearbox.net, netdev@vger.kernel.org,
	bpf@vger.kernel.org
Cc: 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 15:52:55 +0100	[thread overview]
Message-ID: <87mtx34q48.fsf@toke.dk> (raw)
In-Reply-To: <996f1ff7-5891-fd4a-ee3e-fefd7e93879d@intel.com>

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

> On 2021-01-20 13:44, Toke Høiland-Jørgensen wrote:
>> 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.
>>
>
> Yes, *most* of the READ_ONCE(ri->map) are removed, it's pretty much only 
> the bpf_redirect_map(), and as you write, the tracepoints.
>
> The content/element of the map is RCU protected, and actually even the
> map will be around until the XDP processing is complete. Note the
> synchronize_rcu() followed after all bpf_clear_redirect_map() calls.
>
> I'll try to make it clearer in the commit message! Thanks for pointing 
> that out!
>
>> 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?
>>
>
> ...and the bpf_redirect_map() helper. Don't you think the current
> READ_ONCE(ri->map) scheme is more obvious/clear?

Yeah, after your patch we WRITE_ONCE() the pointer in
bpf_redirect_map(), but the only place it is actually *read* is in the
tracepoint. So the only purpose of bpf_clear_redirect_map() is to ensure
that an invalid pointer is not read in the tracepoint function. Which
seems a bit excessive when we could just store the map ID for direct use
in the tracepoint and get rid of bpf_clear_redirect_map() entirely, no?

Besides, from a UX point of view, having the tracepoint display the map
ID even if that map ID is no longer valid seems to me like it makes more
sense than just displaying a map ID of 0 and leaving it up to the user
to figure out that this is because the map was cleared. I mean, at the
time the redirect was made, that *was* the map ID that was used...

Oh, and as you say due to the synchronize_rcu() call in dev_map_free() I
think this whole discussion is superfluous anyway, since it can't
actually happen that the map gets freed between the setting and reading
of ri->map, no?

>> (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? :))
>>
>
> Yeah, good idea! I would guess Hangbin's would go in before this, so I
> would need to adapt.
>
>
> Thanks for taking of look at the series, Toke! Much appreciated!

You're welcome :)

-Toke


  reply	other threads:[~2021-01-20 15:39 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
2021-01-20 13:40     ` Björn Töpel
2021-01-20 14:52       ` Toke Høiland-Jørgensen [this message]
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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