netdev - [PATCH bpf-next 0/2] bpf: safeguard hashtab locking in NMI context

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Date:   Thu, 29 Oct 2020 00:19:23 -0700
From:   Song Liu <songliubraving@...com>
To:     <netdev@...r.kernel.org>, <bpf@...r.kernel.org>
CC:     <kernel-team@...com>, <ast@...nel.org>, <daniel@...earbox.net>,
        <john.fastabend@...il.com>, <kpsingh@...omium.org>,
        Song Liu <songliubraving@...com>
Subject: [PATCH bpf-next 0/2] bpf: safeguard hashtab locking in NMI context

LOCKDEP NMI warning highlighted potential deadlock of hashtab in NMI
context:

[   74.828971] ================================
[   74.828972] WARNING: inconsistent lock state
[   74.828973] 5.9.0-rc8+ #275 Not tainted
[   74.828974] --------------------------------
[   74.828975] inconsistent {INITIAL USE} -> {IN-NMI} usage.
[   74.828976] taskset/1174 [HC2[2]:SC0[0]:HE0:SE1] takes:
[...]
[   74.828999]  Possible unsafe locking scenario:
[   74.828999]
[   74.829000]        CPU0
[   74.829001]        ----
[   74.829001]   lock(&htab->buckets[i].raw_lock);
[   74.829003]   <Interrupt>
[   74.829004]     lock(&htab->buckets[i].raw_lock);

Please refer to patch 1/2 for full trace.

This warning is a false alert, as "INITIAL USE" and "IN-NMI" in the tests
are from different hashtab. On the other hand, in theory, it is possible
to deadlock when a hashtab is access from both non-NMI and NMI context.
Patch 1/2 fixes this false alert by assigning separate lockdep class to
each hashtab. Patch 2/2 introduces map_locked counters, which is similar to
bpf_prog_active counter, to avoid hashtab deadlock in NMI context.

Song Liu (2):
  bpf: use separate lockdep class for each hashtab
  bpf: Avoid hashtab deadlock with map_locked

 kernel/bpf/hashtab.c | 126 +++++++++++++++++++++++++++++++------------
 1 file changed, 92 insertions(+), 34 deletions(-)

--
2.24.1