netdev - Re: [PATCH net v2] ip6mr: Fix lockdep and sparse RCU warnings

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Message-ID: <4e84c550-3328-498d-ad82-8e61b49dc30c@nokia.com>
Date: Tue, 1 Oct 2024 14:50:28 +0200
From: Stefan Wiehler <stefan.wiehler@...ia.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, David Ahern
 <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Petr Malat <oss@...at.biz>
Subject: Re: [PATCH net v2] ip6mr: Fix lockdep and sparse RCU warnings

> OK, but RT6_TABLE_DFLT always exists, ip6mr_get_table(net, RT6_TABLE_DFLT)
> can never fail.
> 
> This is ensured at netns creation, from ip6mr_rules_init()

OK, but nevertheless we need to enter a RCU read-side critical section before
ip6mr_get_table() is called.

> This complex patch adds some code churn, with no clear bug fix ?

We should have probably mentioned the Lockdep-RCU splat that brought this topic
up:

[   48.834645] WARNING: suspicious RCU usage
[   48.834647] 6.1.103-584209f6d5-nokia_sm_x86 #1 Tainted: G S         O
[   48.834649] -----------------------------
[   48.834651] /net/ipv6/ip6mr.c:132 RCU-list traversed in non-reader section!!
[   48.834654]
               other info that might help us debug this:

[   48.834656]
               rcu_scheduler_active = 2, debug_locks = 1
[   48.834658] no locks held by radvd/5777.
[   48.834660]
               stack backtrace:
[   48.834663] CPU: 0 PID: 5777 Comm: radvd Tainted: G S         O       6.1.103-584209f6d5-nokia_sm_x86 #1
[   48.834666] Hardware name: Nokia Asil/Default string, BIOS 0ACNA113 06/07/2024
[   48.834673] Call Trace:
[   48.834674]  <TASK>
[   48.834677]  dump_stack_lvl+0xb7/0xe9
[   48.834687]  lockdep_rcu_suspicious.cold+0x2d/0x64
[   48.834697]  ip6mr_get_table+0x9f/0xb0
[   48.834704]  ip6mr_ioctl+0x50/0x360
[   48.834713]  ? sk_ioctl+0x5f/0x1c0
[   48.834719]  sk_ioctl+0x5f/0x1c0
[   48.834723]  ? find_held_lock+0x2b/0x80
[   48.834731]  sock_do_ioctl+0x7b/0x140
[   48.834737]  ? proc_nr_files+0x30/0x30
[   48.834744]  sock_ioctl+0x1f5/0x360
[   48.834754]  __x64_sys_ioctl+0x8d/0xd0
[   48.834760]  do_syscall_64+0x3c/0x90
[   48.834765]  entry_SYSCALL_64_after_hwframe+0x6e/0xd8
[   48.834769] RIP: 0033:0x7fda5f56e2ac
[   48.834773] Code: 1e fa 48 8d 44 24 08 48 89 54 24 e0 48 89 44 24 c0 48 8d 44 24 d0 48 89 44 24 c8 b8 1 0 00 00 00 c7 44 24 b8 10 00 00 00 0f 05 <3d> 00 f0 ff ff 89 c2 77 0b 89 d0 c3 0f 1f 84 00 00 00 00 00 48 8b
[   48.834776] RSP: 002b:00007fff52d4bda8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[   48.834782] RAX: ffffffffffffffda RBX: 000000000171cd80 RCX: 00007fda5f56e2ac
[   48.834784] RDX: 00007fff52d4bdb0 RSI: 0000000000008913 RDI: 0000000000000003
[   48.834787] RBP: 000000000171cd30 R08: 0000000000000007 R09: 000000000000003c
[   48.834789] R10: 0000000000000004 R11: 0000000000000246 R12: 0000000000000003
[   48.834791] R13: 0000000000000000 R14: 0000000000000004 R15: 000000000040d43c
[   48.834802]  </TASK>

Kind regards,

Stefan