netdev - Re: [PATCH net-next] ipv6: fix a BUG in rt6_get_pcpu

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Message-ID: <20171009170630.vbb26qbuqefg4ady@kafai-mbp.dhcp.thefacebook.com>
Date:   Mon, 9 Oct 2017 10:06:30 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Eric Dumazet <eric.dumazet@...il.com>
CC:     Ido Schimmel <idosch@...lanox.com>, <netdev@...r.kernel.org>,
        <davem@...emloft.net>, <weiwan@...gle.com>, <mlxsw@...lanox.com>
Subject: Re: [PATCH net-next] ipv6: fix a BUG in rt6_get_pcpu_route()

On Mon, Oct 09, 2017 at 04:07:18AM +0000, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> Ido reported following splat and provided a patch.
> 
> [  122.221814] BUG: using smp_processor_id() in preemptible [00000000] code: sshd/2672
> [  122.221845] caller is debug_smp_processor_id+0x17/0x20
> [  122.221866] CPU: 0 PID: 2672 Comm: sshd Not tainted 4.14.0-rc3-idosch-next-custom #639
> [  122.221880] Hardware name: Mellanox Technologies Ltd. MSN2100-CB2FO/SA001017, BIOS 5.6.5 06/07/2016
> [  122.221893] Call Trace:
> [  122.221919]  dump_stack+0xb1/0x10c
> [  122.221946]  ? _atomic_dec_and_lock+0x124/0x124
> [  122.221974]  ? ___ratelimit+0xfe/0x240
> [  122.222020]  check_preemption_disabled+0x173/0x1b0
> [  122.222060]  debug_smp_processor_id+0x17/0x20
> [  122.222083]  ip6_pol_route+0x1482/0x24a0
> ...
> 
> I believe we can simplify this code path a bit, since we no longer
> hold a read_lock and need to release it to avoid a dead lock.
> 
> By disabling BH, we make sure we'll prevent code re-entry and
> rt6_get_pcpu_route()/rt6_make_pcpu_route() run on the same cpu. 
Thanks for fixing it!