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

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Date:   Tue, 14 Feb 2023 20:42:46 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     fhofmann@...udflare.com, davem@...emloft.net, dsahern@...nel.org,
        fred@...udflare.com, kernel-team@...udflare.com, kuba@...nel.org,
        netdev@...r.kernel.org, pabeni@...hat.com, yoshfuji@...ux-ipv6.org
Subject: Re: BUG: using __this_cpu_add() in preemptible in tcp_make_synack()

On Tue, Feb 14, 2023 at 8:23 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From:   Frank Hofmann <fhofmann@...udflare.com>
> Date:   Tue, 14 Feb 2023 17:14:24 +0000
> > 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).
>
> How about converting __SNMP_XXX() only if the kernel is preemptible
> like this ?  Then we can keep the optimised code for other config.
>
> untested & incomplete patch:
>

I would prefer not to.

Let me work on a proper fix.