Re: [Intel-wired-lan] [PATCH iwl-next] ice: allow hot-swapping XDP programs

From: Alexander Lobakin <aleksander.lobakin@intel.com>
To: "Toke Høiland-Jørgensen" <toke@kernel.org>,
	"Maciej Fijalkowski" <maciej.fijalkowski@intel.com>
Cc: netdev@vger.kernel.org, intel-wired-lan@lists.osuosl.org,
	anthony.l.nguyen@intel.com, fred@cloudflare.com,
	magnus.karlsson@intel.com
Subject: Re: [Intel-wired-lan] [PATCH iwl-next] ice: allow hot-swapping XDP programs
Date: Wed, 14 Jun 2023 14:40:07 +0200	[thread overview]
Message-ID: <16f10691-3339-0a18-402a-dc54df5a2e21@intel.com> (raw)
In-Reply-To: <874jnbxmye.fsf@toke.dk>

From: Toke Høiland-Jørgensen <toke@kernel.org>
Date: Tue, 13 Jun 2023 19:59:37 +0200

> Maciej Fijalkowski <maciej.fijalkowski@intel.com> writes:
> 
>> On Tue, Jun 13, 2023 at 05:15:15PM +0200, Alexander Lobakin wrote:
>>> From: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
>>> Date: Tue, 13 Jun 2023 17:10:05 +0200

[...]

>> Since we removed rcu sections from driver sides and given an assumption
>> that local_bh_{dis,en}able() pair serves this purpose now i believe this
>> is safe. Are you aware of:
>>
>> https://lore.kernel.org/bpf/20210624160609.292325-1-toke@redhat.com/

Why [0] then? Added in [1] precisely for the sake of safe XDP prog
access and wasn't removed :s I was relying on that one in my suggestions
and code :D

> 
> As the author of that series, I agree that it's not necessary to add
> additional RCU protection. ice_vsi_assign_bpf_prog() already uses xchg()
> and WRITE_ONCE() which should protect against tearing, and the xdp_prog
> pointer being passed to ice_run_xdp() is a copy residing on the stack,
> so it will only be read once per NAPI cycle anyway (which is in line
> with how most other drivers do it).

What if a NAPI polling cycle is being run on one core while at the very
same moment I'm replacing the XDP prog on another core? Not in terms of
pointer tearing, I see now that this is handled correctly, but in terms
of refcounts? Can't bpf_prog_put() free it while the polling is still
active?

> 
> It *would* be nice to add an __rcu annotation to ice_vsi->xdp_prog and
> ice_rx_ring->xdp_prog (and move to using rcu_dereference(),
> rcu_assign_pointer() etc), but this is more a documentation/static
> checker thing than it's a "correctness of the generated code" thing :)
> 
> -Toke

[0]
https://elixir.bootlin.com/linux/v6.4-rc6/source/drivers/net/ethernet/mellanox/mlx5/core/en_txrx.c#L141
[1]
https://github.com/alobakin/linux/commit/9c25a22dfb00270372224721fed646965420323a

Thanks,
Olek
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