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

KaFai Wan posted 2 patches 2 months, 2 weeks ago
There is a newer version of this series
[PATCH bpf 1/2] bpf: Avoid RCU context warning when unpinning htab with internal structs
Posted by KaFai Wan 2 months, 2 weeks ago 
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@sjtu.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@kernel.org>
Signed-off-by: KaFai Wan <kafai.wan@linux.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,
 };
 
 enum {
-- 
2.43.0
Re: [PATCH bpf 1/2] bpf: Avoid RCU context warning when unpinning htab with internal structs
Posted by Andrii Nakryiko 2 months, 1 week ago 
On Fri, Oct 3, 2025 at 1:47 AM KaFai Wan <kafai.wan@linux.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@sjtu.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@kernel.org>
> Signed-off-by: KaFai Wan <kafai.wan@linux.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
>
Re: [PATCH bpf 1/2] bpf: Avoid RCU context warning when unpinning htab with internal structs
Posted by KaFai Wan 2 months, 1 week ago 
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@linux.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@sjtu.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@kernel.org>
> > Signed-off-by: KaFai Wan <kafai.wan@linux.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

Patchew 2.1-devel-b67e4c2

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