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

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Message-ID: <20230726085003.261112-1-tglozar@redhat.com>
Date: Wed, 26 Jul 2023 10:50:03 +0200
From: tglozar@...hat.com
To: linux-kernel@...r.kernel.org
Cc: john.fastabend@...il.com,
	jakub@...udflare.com,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	netdev@...r.kernel.org,
	bpf@...r.kernel.org,
	Tomas Glozar <tglozar@...hat.com>
Subject: [PATCH RFC net] bpf: sockmap: Remove preempt_disable in sock_map_sk_acquire

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") with no comment on why it is necessary.

Remove preempt_disable to fix BUG in sock_map_update_common on RT.

Signed-off-by: Tomas Glozar <tglozar@...hat.com>
---
 net/core/sock_map.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index 19538d628714..08ab108206bf 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -115,7 +115,6 @@ static void sock_map_sk_acquire(struct sock *sk)
 	__acquires(&sk->sk_lock.slock)
 {
 	lock_sock(sk);
-	preempt_disable();
 	rcu_read_lock();
 }

@@ -123,7 +122,6 @@ static void sock_map_sk_release(struct sock *sk)
 	__releases(&sk->sk_lock.slock)
 {
 	rcu_read_unlock();
-	preempt_enable();
 	release_sock(sk);
 }

base-commit: 22117b3ae6e37d07225653d9ae5ae86b3a54f99c
-- 
2.39.3