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Message-ID: <9671e39c68dd45736dcd1547baba9616e6a07ed2.camel@linux.dev>
Date: Tue, 07 Oct 2025 09:25:54 +0800
From: KaFai Wan <kafai.wan@...ux.dev>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
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 Mon, 2025-10-06 at 16:58 -0700, Andrii Nakryiko wrote:
> 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?
ok, done in v2.
>
> > };
> >
> > enum {
> > --
> > 2.43.0
> >
--
Thanks,
KaFai
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