netdev - Re: [PATCH bpf-next v4 05/19] xdp: add proper __rcu annotations to redirect map entries

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Date:   Thu, 24 Jun 2021 16:52:52 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Cc:     Martin KaFai Lau <kafai@...com>,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH bpf-next v4 05/19] xdp: add proper __rcu annotations to
 redirect map entries

Daniel Borkmann <daniel@...earbox.net> writes:

> On 6/23/21 1:07 PM, Toke Høiland-Jørgensen wrote:
>> XDP_REDIRECT works by a three-step process: the bpf_redirect() and
>> bpf_redirect_map() helpers will lookup the target of the redirect and store
>> it (along with some other metadata) in a per-CPU struct bpf_redirect_info.
>> Next, when the program returns the XDP_REDIRECT return code, the driver
>> will call xdp_do_redirect() which will use the information thus stored to
>> actually enqueue the frame into a bulk queue structure (that differs
>> slightly by map type, but shares the same principle). Finally, before
>> exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will
>> flush all the different bulk queues, thus completing the redirect.
> [...]
>> 
>> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> [...]
>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>> index c5ad7df029ed..b01e266dad9e 100644
>> --- a/include/linux/filter.h
>> +++ b/include/linux/filter.h
>> @@ -762,12 +762,10 @@ DECLARE_BPF_DISPATCHER(xdp)
>>   
>>   static __always_inline u32 bpf_prog_run_xdp(const struct bpf_prog *prog,
>>   					    struct xdp_buff *xdp)
>> -{
>> -	/* Caller needs to hold rcu_read_lock() (!), otherwise program
>> -	 * can be released while still running, or map elements could be
>> -	 * freed early while still having concurrent users. XDP fastpath
>> -	 * already takes rcu_read_lock() when fetching the program, so
>> -	 * it's not necessary here anymore.
>> +
>> +	/* Driver XDP hooks are invoked within a single NAPI poll cycle and thus
>> +	 * under local_bh_disable(), which provides the needed RCU protection
>> +	 * for accessing map entries.
>>   	 */
>>   	return __BPF_PROG_RUN(prog, xdp, BPF_DISPATCHER_FUNC(xdp));
>>   }
>
> I just went over the series to manually fix up merge conflicts in the driver
> patches since they didn't apply cleanly against bpf-next.
>
> But as it turned out that extra work was needless, since you didn't even compile
> test the series before submission, sigh.
>
> Please fix (and only submit compile- & runtime-tested code in future).

Yikes! I was too much in a hurry with to re-submit and neglected to
re-do the compile check before hitting send. Apologies, that was sloppy
of me - I will do better in the future.

Will rebase and send a v5 that doesn't blow up on compile :)

-Toke