netdev - Re: BUG: using __this_cpu_add() in preemptible in tcp_make

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Date:   Tue, 14 Feb 2023 17:14:24 +0000
From:   Frank Hofmann <fhofmann@...udflare.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Frederick Lawler <fred@...udflare.com>, netdev@...r.kernel.org,
        kernel-team@...udflare.com, davem@...emloft.net,
        yoshfuji@...ux-ipv6.org, dsahern@...nel.org, kuba@...nel.org,
        pabeni@...hat.com
Subject: Re: BUG: using __this_cpu_add() in preemptible in tcp_make_synack()

Hi Eric,

On Mon, Jan 23, 2023 at 3:49 PM 'Eric Dumazet' via
kernel-team+notifications <kernel-team@...udflare.com> wrote:
>
> > On 1/18/23 11:07 AM, Eric Dumazet wrote:
> [ ... ]
> > > Thanks for the report
> > >
> > > I guess this part has been missed in commit 0a375c822497ed6a
> > >
> > > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> > > index 71d01cf3c13eb4bd3d314ef140568d2ffd6a499e..ba839e441450f195012a8d77cb9e5ed956962d2f
> > > 100644
> > > --- a/net/ipv4/tcp_output.c
> > > +++ b/net/ipv4/tcp_output.c
> > > @@ -3605,7 +3605,7 @@ struct sk_buff *tcp_make_synack(const struct
> > > sock *sk, struct dst_entry *dst,
[ ... ]

we're still seeing this with a preempt-enabled kernel, in
tcp_check_req() though, like:

BUG: using __this_cpu_add() in preemptible [00000000] code: nginx-ssl/186233
caller is tcp_check_req+0x49a/0x660
CPU: 58 PID: 186233 Comm: nginx-ssl Kdump: loaded Tainted: G
O       6.1.8-cloudflare-2023.1.16 #1
Hardware name: ...
Call Trace:
 <TASK>
 dump_stack_lvl+0x34/0x48
 check_preemption_disabled+0xdd/0xe0
 tcp_check_req+0x49a/0x660
 tcp_rcv_state_process+0xa3/0x1020
 ? tcp_sendmsg_locked+0x2a4/0xc50
 tcp_v4_do_rcv+0xc6/0x280
 __release_sock+0xb4/0xc0
 release_sock+0x2b/0x90
 tcp_sendmsg+0x33/0x40
 sock_sendmsg+0x5b/0x70
 sock_write_iter+0x97/0x100
 vfs_write+0x330/0x3d0
 ksys_write+0xab/0xe0
 ? syscall_trace_enter.constprop.0+0x164/0x170
 do_syscall_64+0x3b/0x90
 entry_SYSCALL_64_after_hwframe+0x4b/0xb5

There's a notable number of "__"-marked stats updates in
tcp_check_req(); I can't claim to understand the code well enough if
all would have to be changed.
The occurence is infrequent (we see about two a week).

Thanks for any pointers!
Frank Hofmann