Subject: Re: [PATCH net] bpf: sockmap: Remove preempt_disable in sock_map_sk_acquire

From: Jiri Olsa <[email-masked]>
Date: Fri, 28 Jul 2023 13:48:34 +0200

On Fri, Jul 28, 2023 at 08:44:11AM +0200, [email-masked] wrote:
> From: Tomas Glozar <[email-masked]>
> 
> 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.

FYI, I'm not sure it's related but I started to see following splat recently:

[  189.360689][  T658] =============================
[  189.361149][  T658] [ BUG: Invalid wait context ]
[  189.361588][  T658] 6.5.0-rc2+ #589 Tainted: G           OE     
[  189.362174][  T658] -----------------------------
[  189.362660][  T658] test_progs/658 is trying to lock:
[  189.363176][  T658] ffff8881702652b8 (&psock->link_lock){....}-{3:3}, at: sock_map_update_common+0x1c4/0x340
[  189.364152][  T658] other info that might help us debug this:
[  189.364689][  T658] context-{5:5}
[  189.365021][  T658] 3 locks held by test_progs/658:
[  189.365508][  T658]  #0: ffff888177611a80 (sk_lock-AF_INET){+.+.}-{0:0}, at: sock_map_update_elem_sys+0x82/0x260
[  189.366503][  T658]  #1: ffffffff835a3180 (rcu_read_lock){....}-{1:3}, at: sock_map_update_elem_sys+0x78/0x260
[  189.367470][  T658]  #2: ffff88816cf19240 (&stab->lock){+...}-{2:2}, at: sock_map_update_common+0x12a/0x340
[  189.368420][  T658] stack backtrace:
[  189.368806][  T658] CPU: 0 PID: 658 Comm: test_progs Tainted: G           OE      6.5.0-rc2+ #589 98af30b3c42d747b51da05f1d0e4899e394be6c9
[  189.369889][  T658] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-1.fc38 04/01/2014
[  189.370736][  T658] Call Trace:
[  189.371063][  T658]  <TASK>
[  189.371365][  T658]  dump_stack_lvl+0xb2/0x120
[  189.371798][  T658]  __lock_acquire+0x9ad/0x2470
[  189.372243][  T658]  ? lock_acquire+0x104/0x350
[  189.372680][  T658]  lock_acquire+0x104/0x350
[  189.373104][  T658]  ? sock_map_update_common+0x1c4/0x340
[  189.373615][  T658]  ? find_held_lock+0x32/0x90
[  189.374074][  T658]  ? sock_map_update_common+0x12a/0x340
[  189.374587][  T658]  _raw_spin_lock_bh+0x38/0x80
[  189.375060][  T658]  ? sock_map_update_common+0x1c4/0x340
[  189.375571][  T658]  sock_map_update_common+0x1c4/0x340
[  189.376118][  T658]  sock_map_update_elem_sys+0x184/0x260
[  189.376704][  T658]  __sys_bpf+0x181f/0x2840
[  189.377147][  T658]  __x64_sys_bpf+0x1a/0x30
[  189.377556][  T658]  do_syscall_64+0x38/0x90
[  189.377980][  T658]  entry_SYSCALL_64_after_hwframe+0x6e/0xd8
[  189.378473][  T658] RIP: 0033:0x7fe52f47ab5d

the patch did not help with that

jirka

> 
> Signed-off-by: Tomas Glozar <[email-masked]>
> ---
>  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);
>  }
>  
> -- 
> 2.39.3
> 
>