netdev - Re: [PATCH net] bpf: sockmap: Remove preempt_disable in sock_map_sk

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Date: Mon, 31 Jul 2023 20:58:53 -0700
From: John Fastabend <john.fastabend@...il.com>
To: Jakub Sitnicki <jakub@...udflare.com>, 
 tglozar@...hat.com
Cc: linux-kernel@...r.kernel.org, 
 john.fastabend@...il.com, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 netdev@...r.kernel.org, 
 bpf@...r.kernel.org
Subject: Re: [PATCH net] bpf: sockmap: Remove preempt_disable in
 sock_map_sk_acquire

Jakub Sitnicki wrote:
> 
> On Fri, Jul 28, 2023 at 08:44 AM +02, tglozar@...hat.com wrote:
> > From: Tomas Glozar <tglozar@...hat.com>
> >
> > Disabling preemption in sock_map_sk_acquire conflicts with GFP_ATOMIC
> > allocation later in sk_psock_init_link on PREEMPT_RT kernels, since
> > GFP_ATOMIC might sleep on RT (see bpf: Make BPF and PREEMPT_RT co-exist
> > patchset notes for details).
> >
> > This causes calling bpf_map_update_elem on BPF_MAP_TYPE_SOCKMAP maps to
> > BUG (sleeping function called from invalid context) on RT kernels.
> >
> > preempt_disable was introduced together with lock_sk and rcu_read_lock
> > in commit 99ba2b5aba24e ("bpf: sockhash, disallow bpf_tcp_close and update
> > in parallel"), probably to match disabled migration of BPF programs, and
> > is no longer necessary.
> >
> > Remove preempt_disable to fix BUG in sock_map_update_common on RT.
> >
> > Signed-off-by: Tomas Glozar <tglozar@...hat.com>
> > ---
> 
> We disable softirq and hold a spin lock when modifying the map/hash in
> sock_{map,hash}_update_common so this LGTM:
> 

Agree.

> Reviewed-by: Jakub Sitnicki <jakub@...udflare.com>

Reviewed-by: John Fastabend <john.fastabend@...il.com>

> 
> You might want some extra tags:
> 
> Link: https://lore.kernel.org/all/20200224140131.461979697@linutronix.de/
> Fixes: 99ba2b5aba24 ("bpf: sockhash, disallow bpf_tcp_close and update in parallel")