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

[KaFai Wan]

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@xxxxxxxxx> 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@xxxxxxxxxxx>
> > Closes:
> > https://lore.kernel.org/all/1444123482.1827743.1750996347470.JavaMail.zimbra@xxxxxxxxxxx/
> > Fixes: 68134668c17f ("bpf: Add map side support for bpf timers.")
> > Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> > Signed-off-by: KaFai Wan <kafai.wan@xxxxxxxxx>
> > ---
> >  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