Subject: Re: [PATCH bpf v2 3/4] bpf, sockmap: Adapt for the af_unix-specific lock

From: Martin KaFai Lau <[email-masked]>
Date: Mon, 9 Feb 2026 12:17:17 -0800

On 2/8/26 9:14 AM, Michal Luczaj wrote:
> On 2/7/26 23:00, Kuniyuki Iwashima wrote:
>> On Sat, Feb 7, 2026 at 6:35 AM Michal Luczaj <[email-masked]> wrote:
>>> This patch also happens to fix a deadlock that may occur when
>>> bpf_iter_unix_seq_show()'s lock_sock_fast() takes the fast path and the
>>> iter prog attempts to update a sockmap. Which ends up spinning at
>>> sock_map_update_elem()'s bh_lock_sock():
>>
>> Hmm.. this seems to be a more general problem for
>> bpf iter vs sockmap.  bpf_iter_{tcp,udp}_seq_show() also
>> hold lock_sock(),  where this patch's solution does not help.
>> We need to resolve this regardless of socket family.
> 
> I don't see any deadlocks there. Note that I've mentioned lock_sock_fast()
> fast path was a problem, not lock_sock().

For the tcp/udp, I think the bpf_iter should be fine: lock_sock() in 
seq_show and bh_lock_sock() in map_update. It seems redundant though.

 From looking at may_update_sockmap(), other bpf progs (e.g., tc) can do 
map_update also. On those paths, I am not sure why 
sock_map_update_elem() does not need to check "!sock_owned_by_user(sk)". 
If it is indeed an issue, it probably needs to be addressed separately.

It should also be helpful to be consistent with tcp/udp iter and use 
lock_sock() instead of lock_sock_fast() in bpf_iter_unix_seq_show().