From: "Toke Høiland-Jørgensen" <toke@redhat•com>
To: Daniel Borkmann <daniel@iogearbox•net>,
Alexei Starovoitov <alexei.starovoitov@gmail•com>
Cc: bpf <bpf@vger•kernel.org>,
Network Development <netdev@vger•kernel.org>,
Martin KaFai Lau <kafai@fb•com>,
Hangbin Liu <liuhangbin@gmail•com>,
Jesper Dangaard Brouer <brouer@redhat•com>,
Magnus Karlsson <magnus.karlsson@gmail•com>,
"Paul E . McKenney" <paulmck@kernel•org>
Subject: Re: [PATCH bpf-next 02/17] bpf: allow RCU-protected lookups to happen from bh context
Date: Fri, 11 Jun 2021 00:27:52 +0200 [thread overview]
Message-ID: <874ke5we1j.fsf@toke.dk> (raw)
In-Reply-To: <c5192ab3-1c05-8679-79f2-59d98299095b@iogearbox.net>
Daniel Borkmann <daniel@iogearbox•net> writes:
> Hi Paul,
>
> On 6/10/21 8:38 PM, Alexei Starovoitov wrote:
>> On Wed, Jun 9, 2021 at 7:24 AM Toke Høiland-Jørgensen <toke@redhat•com> wrote:
>>>
>>> XDP programs are called from a NAPI poll context, which means the RCU
>>> reference liveness is ensured by local_bh_disable(). Add
>>> rcu_read_lock_bh_held() as a condition to the RCU checks for map lookups so
>>> lockdep understands that the dereferences are safe from inside *either* an
>>> rcu_read_lock() section *or* a local_bh_disable() section. This is done in
>>> preparation for removing the redundant rcu_read_lock()s from the drivers.
>>>
>>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat•com>
>>> ---
>>> kernel/bpf/hashtab.c | 21 ++++++++++++++-------
>>> kernel/bpf/helpers.c | 6 +++---
>>> kernel/bpf/lpm_trie.c | 6 ++++--
>>> 3 files changed, 21 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
>>> index 6f6681b07364..72c58cc516a3 100644
>>> --- a/kernel/bpf/hashtab.c
>>> +++ b/kernel/bpf/hashtab.c
>>> @@ -596,7 +596,8 @@ static void *__htab_map_lookup_elem(struct bpf_map *map, void *key)
>>> struct htab_elem *l;
>>> u32 hash, key_size;
>>>
>>> - WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
>>> + WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held() &&
>>> + !rcu_read_lock_bh_held());
>>
>> It's not clear to me whether rcu_read_lock_held() is still needed.
>> All comments sound like rcu_read_lock_bh_held() is a superset of rcu
>> that includes bh.
>> But reading rcu source code it looks like RCU_BH is its own rcu flavor...
>> which is confusing.
>
> The series is a bit confusing to me as well. I recall we had a discussion with
> Paul, but it was back in 2016 aka very early days of XDP to get some clarifications
> about RCU vs RCU-bh flavour on this. Paul, given the series in here, I assume the
> below is not true anymore, and in this case (since we're removing rcu_read_lock()
> from drivers), the RCU-bh acts as a real superset?
>
> Back then from your clarifications this was not the case:
>
> On Mon, Jul 25, 2016 at 11:26:02AM -0700, Alexei Starovoitov wrote:
> > On Mon, Jul 25, 2016 at 11:03 AM, Paul E. McKenney
> > <paulmck@linux•vnet.ibm.com> wrote:
> [...]
> >>> The crux of the question is whether a particular driver rx handler, when
> >>> called from __do_softirq, needs to add an additional rcu_read_lock or
> >>> whether it can rely on the mechanics of softirq.
> >>
> >> If it was rcu_read_lock_bh(), you could.
> >>
> >> But you didn't say rcu_read_lock_bh(), you instead said rcu_read_lock(),
> >> which means that you absolutely cannot rely on softirq semantics.
> >>
> >> In particular, in CONFIG_PREEMPT=y kernels, rcu_preempt_check_callbacks()
> >> will notice that there is no rcu_read_lock() in effect and report
> >> a quiescent state for that CPU. Because rcu_preempt_check_callbacks()
> >> is invoked from the scheduling-clock interrupt, it absolutely can
> >> execute during do_softirq(), and therefore being in softirq context
> >> in no way provides rcu_read_lock()-style protection.
> >>
> >> Now, Alexei's question was for CONFIG_PREEMPT=n kernels. However, in
> >> that case, rcu_read_lock() and rcu_read_unlock() generate no code
> >> in recent production kernels, so there is no performance penalty for
> >> using them. (In older kernels, they implied a barrier().)
> >>
> >> So either way, with or without CONFIG_PREEMPT, you should use
> >> rcu_read_lock() to get RCU protection.
> >>
> >> One alternative might be to switch to rcu_read_lock_bh(), but that
> >> will add local_disable_bh() overhead to your read paths.
> >>
> >> Does that help, or am I missing the point of the question?
> >
> > thanks a lot for explanation.
>
> Glad you liked it!
>
> > I mistakenly assumed that _bh variants are 'stronger' and
> > act as inclusive, but sounds like they're completely orthogonal
> > especially with preempt_rcu=y.
>
> Yes, they are pretty much orthogonal.
>
> > With preempt_rcu=n and preempt=y, it would be the case, since
> > bh disables preemption and rcu_read_lock does the same as well,
> > right? Of course, the code shouldn't be relying on that, so we
> > have to fix our stuff.
>
> Indeed, especially given that the kernel currently won't allow you
> to configure CONFIG_PREEMPT_RCU=n and CONFIG_PREEMPT=y. If it does,
> please let me know, as that would be a bug that needs to be fixed.
> (For one thing, I do not test that combination.)
>
> Thanx, Paul
>
> And now, fast-forward again to 2021 ... :)
We covered this in the thread I linked from the cover letter.
Specifically, this seems to have been a change from v4.20, see Paul's
reply here:
https://lore.kernel.org/bpf/20210417002301.GO4212@paulmck-ThinkPad-P17-Gen-1/
and the follow-up covering -rt here:
https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/
-Toke
next prev parent reply other threads:[~2021-06-10 22:27 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-09 10:33 [PATCH bpf-next 00/17] Clean up and document RCU-based object protection for XDP_REDIRECT Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 01/17] rcu: Create an unrcu_pointer() to remove __rcu from a pointer Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 02/17] bpf: allow RCU-protected lookups to happen from bh context Toke Høiland-Jørgensen
2021-06-10 18:38 ` Alexei Starovoitov
2021-06-10 21:24 ` Daniel Borkmann
2021-06-10 22:27 ` Toke Høiland-Jørgensen [this message]
2021-06-10 19:33 ` Martin KaFai Lau
2021-06-09 10:33 ` [PATCH bpf-next 03/17] dev: add rcu_read_lock_bh_held() as a valid check when getting a RCU dev ref Toke Høiland-Jørgensen
2021-06-10 19:37 ` Martin KaFai Lau
2021-06-10 23:05 ` Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 04/17] xdp: add proper __rcu annotations to redirect map entries Toke Høiland-Jørgensen
2021-06-10 21:09 ` Martin KaFai Lau
2021-06-10 23:19 ` Toke Høiland-Jørgensen
2021-06-10 23:32 ` Martin KaFai Lau
2021-06-10 23:41 ` Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 05/17] ena: remove rcu_read_lock() around XDP program invocation Toke Høiland-Jørgensen
2021-06-09 13:57 ` Paul E. McKenney
2021-06-09 10:33 ` [PATCH bpf-next 06/17] bnxt: " Toke Høiland-Jørgensen
2021-06-09 13:58 ` Paul E. McKenney
2021-06-10 8:47 ` Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 07/17] thunderx: " Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 08/17] freescale: " Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 09/17] net: intel: " Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 10/17] marvell: " Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 11/17] mlx4: " Toke Høiland-Jørgensen
2021-06-10 7:10 ` Tariq Toukan
2021-06-09 10:33 ` [PATCH bpf-next 12/17] nfp: " Toke Høiland-Jørgensen
2021-06-11 16:30 ` Simon Horman
2021-06-09 10:33 ` [PATCH bpf-next 13/17] qede: " Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 14/17] sfc: " Toke Høiland-Jørgensen
2021-06-09 12:15 ` Edward Cree
2021-06-09 10:33 ` [PATCH bpf-next 15/17] netsec: " Toke Høiland-Jørgensen
2021-06-10 5:30 ` Ilias Apalodimas
2021-06-09 10:33 ` [PATCH bpf-next 16/17] stmmac: " Toke Høiland-Jørgensen
2021-06-09 10:33 ` [PATCH bpf-next 17/17] net: ti: " Toke Høiland-Jørgensen
2021-06-09 17:04 ` Grygorii Strashko
2021-06-10 0:18 ` [PATCH bpf-next 00/17] Clean up and document RCU-based object protection for XDP_REDIRECT Yonghong Song
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