On Fri, Jul 28, 2023 at 08:44 AM +02, tglozar@redhat.com wrote:

From: Tomas Glozar <tglozar@redhat.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@redhat.com>
---

We disable softirq and hold a spin lock when modifying the map/hash in
sock_{map,hash}_update_common so this LGTM:

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

You might want some extra tags:

Link: https://lore.kernel.org/all/20200224140131.461979697@linutronix.de/ (local)
Fixes: 99ba2b5aba24 ("bpf: sockhash, disallow bpf_tcp_close and update in parallel")