linux-kernel - Re: [PATCH bpf 1/2] bpf: Avoid RCU context warning when unpinning htab with internal structs

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Message-ID: <CAEf4BzZhGEgW82gweZtW1Cp5L1_pafUwML8jMifBvjzBtnWWeA@mail.gmail.com>
Date: Mon, 6 Oct 2025 16:58:54 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: KaFai Wan <kafai.wan@...ux.dev>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, 
	martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org, 
	sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, shuah@...nel.org, 
	toke@...hat.com, linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, Le Chen <tom2cat@...u.edu.cn>
Subject: Re: [PATCH bpf 1/2] bpf: Avoid RCU context warning when unpinning
 htab with internal structs

On Fri, Oct 3, 2025 at 1:47 AM KaFai Wan <kafai.wan@...ux.dev> wrote:
>
> When unpinning a BPF hash table (htab or htab_lru) that contains internal
> structures (timer, workqueue, or task_work) in its values, a BUG warning
> is triggered:
>  BUG: sleeping function called from invalid context at kernel/bpf/hashtab.c:244
>  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 14, name: ksoftirqd/0
>  ...
>
> The issue arises from the interaction between BPF object unpinning and
> RCU callback mechanisms:
> 1. BPF object unpinning uses ->free_inode() which schedules cleanup via
>    call_rcu(), deferring the actual freeing to an RCU callback that
>    executes within the RCU_SOFTIRQ context.
> 2. During cleanup of hash tables containing internal structures,
>    htab_map_free_internal_structs() is invoked, which includes
>    cond_resched() or cond_resched_rcu() calls to yield the CPU during
>    potentially long operations.
>
> However, cond_resched() or cond_resched_rcu() cannot be safely called from
> atomic RCU softirq context, leading to the BUG warning when attempting
> to reschedule.
>
> Fix this by changing from ->free_inode() to ->destroy_inode() for BPF
> objects (prog, map, link). This allows direct inode freeing without
> RCU callback scheduling, avoiding the invalid context warning.
>
> Reported-by: Le Chen <tom2cat@...u.edu.cn>
> Closes: https://lore.kernel.org/all/1444123482.1827743.1750996347470.JavaMail.zimbra@sjtu.edu.cn/
> Fixes: 68134668c17f ("bpf: Add map side support for bpf timers.")
> Suggested-by: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: KaFai Wan <kafai.wan@...ux.dev>
> ---
>  kernel/bpf/inode.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
> index f90bdcc0a047..65c2a71d7de1 100644
> --- a/kernel/bpf/inode.c
> +++ b/kernel/bpf/inode.c
> @@ -790,7 +790,7 @@ const struct super_operations bpf_super_ops = {
>         .statfs         = simple_statfs,
>         .drop_inode     = inode_just_drop,
>         .show_options   = bpf_show_options,
> -       .free_inode     = bpf_free_inode,
> +       .destroy_inode  = bpf_free_inode,

s/bpf_free_inode/bpf_destroy_inode/ then?

>  };
>
>  enum {
> --
> 2.43.0
>