[PATCH bpf 0/2] bpf: Avoid RCU context warning when unpinning htab with internal structs

[PATCH bpf 0/2] bpf: Avoid RCU context warning when unpinning htab with internal structs

[KaFai Wan]

This small patchset is about avoid RCU context warning when unpinning
htab with internal structs (timer, workqueue, or task_work).

---
KaFai Wan (2):
  bpf: Avoid RCU context warning when unpinning htab with internal
    structs
  selftests/bpf: Add test for unpinning htab with internal timer struct

 kernel/bpf/inode.c                            |  2 +-
 .../selftests/bpf/prog_tests/pinning_htab.c   | 37 +++++++++++++++++++
 .../selftests/bpf/progs/test_pinning_htab.c   | 25 +++++++++++++
 3 files changed, 63 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/pinning_htab.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_pinning_htab.c

-- 
2.43.0

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

[KaFai Wan]

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

[PATCH bpf 2/2] selftests/bpf: Add test for unpinning htab with internal timer struct

[KaFai Wan]

Add test to verify that unpinning hash tables containing internal timer
structures does not trigger context warnings.

Each subtest (timer_prealloc and timer_no_prealloc) can trigger the
context warning when unpinning, but the warning cannot be triggered
twice within a short time interval (a HZ), which is expected behavior.

Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
---
 .../selftests/bpf/prog_tests/pinning_htab.c   | 37 +++++++++++++++++++
 .../selftests/bpf/progs/test_pinning_htab.c   | 25 +++++++++++++
 2 files changed, 62 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/pinning_htab.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_pinning_htab.c

diff --git a/tools/testing/selftests/bpf/prog_tests/pinning_htab.c b/tools/testing/selftests/bpf/prog_tests/pinning_htab.c
new file mode 100644
index 000000000000..fc804bb87b26
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/pinning_htab.c
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include "test_pinning_htab.skel.h"
+
+static void unpin_map(const char *map_name, const char *pin_path)
+{
+	struct test_pinning_htab *skel;
+	struct bpf_map *map;
+	int err;
+
+	skel = test_pinning_htab__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel open_and_load"))
+		return;
+
+	map = bpf_object__find_map_by_name(skel->obj, map_name);
+	if (!ASSERT_OK_PTR(map, "bpf_object__find_map_by_name"))
+		goto out;
+
+	err = bpf_map__pin(map, pin_path);
+	if (!ASSERT_OK(err, "bpf_map__pin"))
+		goto out;
+
+	err = bpf_map__unpin(map, pin_path);
+	if (!ASSERT_OK(err, "bpf_map__unpin"))
+		goto out;
+out:
+	test_pinning_htab__destroy(skel);
+}
+
+void test_pinning_htab(void)
+{
+	if (test__start_subtest("timer_prealloc"))
+		unpin_map("timer_prealloc", "/sys/fs/bpf/timer_prealloc");
+	if (test__start_subtest("timer_no_prealloc"))
+		unpin_map("timer_no_prealloc", "/sys/fs/bpf/timer_no_prealloc");
+}
diff --git a/tools/testing/selftests/bpf/progs/test_pinning_htab.c b/tools/testing/selftests/bpf/progs/test_pinning_htab.c
new file mode 100644
index 000000000000..ae227930c73c
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_pinning_htab.c
@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct timer_val {
+	struct bpf_timer timer;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__type(key, __u32);
+	__type(value, struct timer_val);
+	__uint(max_entries, 1);
+} timer_prealloc SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__type(key, __u32);
+	__type(value, struct timer_val);
+	__uint(max_entries, 1);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+} timer_no_prealloc SEC(".maps");
-- 
2.43.0

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

[Andrii Nakryiko]

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

[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@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