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Message-ID: <20251002225356.1505480-3-ameryhung@gmail.com>
Date: Thu, 2 Oct 2025 15:53:41 -0700
From: Amery Hung <ameryhung@...il.com>
To: bpf@...r.kernel.org
Cc: netdev@...r.kernel.org,
alexei.starovoitov@...il.com,
andrii@...nel.org,
daniel@...earbox.net,
memxor@...il.com,
martin.lau@...nel.org,
kpsingh@...nel.org,
yonghong.song@...ux.dev,
song@...nel.org,
haoluo@...gle.com,
ameryhung@...il.com,
kernel-team@...a.com
Subject: [RFC PATCH bpf-next v2 02/12] bpf: Convert bpf_selem_unlink_map to failable
To prepare for changing bpf_local_storage_map_bucket::lock to rqspinlock,
convert bpf_selem_unlink_map() to failable. It still always succeeds and
returns 0 for now.
Since some operations updating local storage cannot fail in the middle,
open-code bpf_selem_unlink_map() to take the b->lock before the
operation. There are two such locations:
- bpf_local_storage_alloc()
The first selem will be unlinked from smap if cmpxchg owner_storage_ptr
fails, which should not fail. Therefore, hold b->lock when linking
until allocation complete. Helpers that assume b->lock is held by
callers are introduced: bpf_selem_link_map_nolock() and
bpf_selem_unlink_map_nolock().
- bpf_local_storage_update()
The three step update process: link_map(new_selem),
link_storage(new_selem), and unlink_map(old_selem) should not fail in
the middle.
In bpf_selem_unlink(), bpf_selem_unlink_map() and
bpf_selem_unlink_storage() should either all succeed or fail as a whole
instead of failing in the middle. So, return if unlink_map() failed.
In bpf_local_storage_destroy(), since it cannot deadlock with itself or
bpf_local_storage_map_free() who the function might be racing with,
retry if bpf_selem_unlink_map() fails due to rqspinlock returning
errors.
Signed-off-by: Amery Hung <ameryhung@...il.com>
---
kernel/bpf/bpf_local_storage.c | 54 ++++++++++++++++++++++++++++------
1 file changed, 45 insertions(+), 9 deletions(-)
diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
index e4a7cd33b455..cbccf6b77f10 100644
--- a/kernel/bpf/bpf_local_storage.c
+++ b/kernel/bpf/bpf_local_storage.c
@@ -409,7 +409,7 @@ void bpf_selem_link_storage_nolock(struct bpf_local_storage *local_storage,
hlist_add_head_rcu(&selem->snode, &local_storage->list);
}
-static void bpf_selem_unlink_map(struct bpf_local_storage_elem *selem)
+static int bpf_selem_unlink_map(struct bpf_local_storage_elem *selem)
{
struct bpf_local_storage *local_storage;
struct bpf_local_storage_map *smap;
@@ -418,7 +418,7 @@ static void bpf_selem_unlink_map(struct bpf_local_storage_elem *selem)
if (unlikely(!selem_linked_to_map_lockless(selem)))
/* selem has already be unlinked from smap */
- return;
+ return 0;
local_storage = rcu_dereference_check(selem->local_storage,
bpf_rcu_lock_held());
@@ -428,6 +428,14 @@ static void bpf_selem_unlink_map(struct bpf_local_storage_elem *selem)
if (likely(selem_linked_to_map(selem)))
hlist_del_init_rcu(&selem->map_node);
raw_spin_unlock_irqrestore(&b->lock, flags);
+
+ return 0;
+}
+
+static void bpf_selem_unlink_map_nolock(struct bpf_local_storage_elem *selem)
+{
+ if (likely(selem_linked_to_map(selem)))
+ hlist_del_init_rcu(&selem->map_node);
}
void bpf_selem_link_map(struct bpf_local_storage_map *smap,
@@ -446,13 +454,26 @@ void bpf_selem_link_map(struct bpf_local_storage_map *smap,
raw_spin_unlock_irqrestore(&b->lock, flags);
}
+static void bpf_selem_link_map_nolock(struct bpf_local_storage_map *smap,
+ struct bpf_local_storage_elem *selem,
+ struct bpf_local_storage_map_bucket *b)
+{
+ RCU_INIT_POINTER(SDATA(selem)->smap, smap);
+ hlist_add_head_rcu(&selem->map_node, &b->list);
+}
+
void bpf_selem_unlink(struct bpf_local_storage_elem *selem, bool reuse_now)
{
+ int err;
+
/* Always unlink from map before unlinking from local_storage
* because selem will be freed after successfully unlinked from
* the local_storage.
*/
- bpf_selem_unlink_map(selem);
+ err = bpf_selem_unlink_map(selem);
+ if (err)
+ return;
+
bpf_selem_unlink_storage(selem, reuse_now);
}
@@ -494,6 +515,8 @@ int bpf_local_storage_alloc(void *owner,
{
struct bpf_local_storage *prev_storage, *storage;
struct bpf_local_storage **owner_storage_ptr;
+ struct bpf_local_storage_map_bucket *b;
+ unsigned long flags;
int err;
err = mem_charge(smap, owner, sizeof(*storage));
@@ -516,7 +539,10 @@ int bpf_local_storage_alloc(void *owner,
storage->owner = owner;
bpf_selem_link_storage_nolock(storage, first_selem);
- bpf_selem_link_map(smap, first_selem);
+
+ b = select_bucket(smap, storage);
+ raw_spin_lock_irqsave(&b->lock, flags);
+ bpf_selem_link_map_nolock(smap, first_selem, b);
owner_storage_ptr =
(struct bpf_local_storage **)owner_storage(smap, owner);
@@ -532,7 +558,8 @@ int bpf_local_storage_alloc(void *owner,
*/
prev_storage = cmpxchg(owner_storage_ptr, NULL, storage);
if (unlikely(prev_storage)) {
- bpf_selem_unlink_map(first_selem);
+ bpf_selem_unlink_map_nolock(first_selem);
+ raw_spin_unlock_irqrestore(&b->lock, flags);
err = -EAGAIN;
goto uncharge;
@@ -546,6 +573,7 @@ int bpf_local_storage_alloc(void *owner,
* bucket->list under rcu_read_lock().
*/
}
+ raw_spin_unlock_irqrestore(&b->lock, flags);
return 0;
@@ -567,8 +595,9 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
struct bpf_local_storage_data *old_sdata = NULL;
struct bpf_local_storage_elem *alloc_selem, *selem = NULL;
struct bpf_local_storage *local_storage;
+ struct bpf_local_storage_map_bucket *b;
HLIST_HEAD(old_selem_free_list);
- unsigned long flags;
+ unsigned long flags, b_flags;
int err;
/* BPF_EXIST and BPF_NOEXIST cannot be both set */
@@ -652,20 +681,26 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
goto unlock;
}
+ b = select_bucket(smap, local_storage);
+
+ raw_spin_lock_irqsave(&b->lock, b_flags);
+
alloc_selem = NULL;
/* First, link the new selem to the map */
- bpf_selem_link_map(smap, selem);
+ bpf_selem_link_map_nolock(smap, selem, b);
/* Second, link (and publish) the new selem to local_storage */
bpf_selem_link_storage_nolock(local_storage, selem);
/* Third, remove old selem, SELEM(old_sdata) */
if (old_sdata) {
- bpf_selem_unlink_map(SELEM(old_sdata));
+ bpf_selem_unlink_map_nolock(SELEM(old_sdata));
bpf_selem_unlink_storage_nolock(local_storage, SELEM(old_sdata),
true, &old_selem_free_list);
}
+ raw_spin_unlock_irqrestore(&b->lock, b_flags);
+
unlock:
raw_spin_unlock_irqrestore(&local_storage->lock, flags);
bpf_selem_free_list(&old_selem_free_list, false);
@@ -743,6 +778,7 @@ void bpf_local_storage_destroy(struct bpf_local_storage *local_storage)
HLIST_HEAD(free_selem_list);
struct hlist_node *n;
unsigned long flags;
+ int err;
storage_smap = rcu_dereference_check(local_storage->smap, bpf_rcu_lock_held());
bpf_ma = check_storage_bpf_ma(local_storage, storage_smap, NULL);
@@ -761,7 +797,7 @@ void bpf_local_storage_destroy(struct bpf_local_storage *local_storage)
/* Always unlink from map before unlinking from
* local_storage.
*/
- bpf_selem_unlink_map(selem);
+ while (bpf_selem_unlink_map(selem));
/* If local_storage list has only one element, the
* bpf_selem_unlink_storage_nolock() will return true.
* Otherwise, it will return false. The current loop iteration
--
2.47.3
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