[PATCH bpf-next 4/9] bpf: Avoid taking spinlock in bpf_task_storage_get if potential deadlock is detected

Martin KaFai Lau
2022-10-25T18:46:20Z

bpf_task_storage_get() does a lookup and optionally inserts
new data if BPF_LOCAL_STORAGE_GET_F_CREATE is present.

During lookup, it will cache the lookup result and caching requires to
acquire a spinlock. When potential deadlock is detected (by the
bpf_task_storage_busy pcpu-counter added in
commit bc235cdb423a ("bpf: Prevent deadlock from recursive bpf_task_storage_[get|delete]")),
the current behavior is returning NULL immediately to avoid deadlock. It is
too pessimistic. This patch will go ahead to do a lookup (which is a
lockless operation) but it will avoid caching it in order to avoid
acquiring the spinlock.

When lookup fails to find the data and BPF_LOCAL_STORAGE_GET_F_CREATE
is set, an insertion is needed and this requires acquiring a spinlock.
This patch will still return NULL when a potential deadlock is detected.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
---
kernel/bpf/bpf_local_storage.c | 1 +
kernel/bpf/bpf_task_storage.c | 15 ++++++++-------
2 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
index 9dc6de1cf185..781d14167140 100644
--- a/kernel/bpf/bpf_local_storage.c
+++ b/kernel/bpf/bpf_local_storage.c
@@ -242,6 +242,7 @@ void bpf_selem_unlink(struct bpf_local_storage_elem *selem, bool use_trace_rcu)
__bpf_selem_unlink_storage(selem, use_trace_rcu);
}

+/* If cacheit_lockit is false, this lookup function is lockless */
struct bpf_local_storage_data *
bpf_local_storage_lookup(struct bpf_local_storage *local_storage,
struct bpf_local_storage_map *smap,
diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_storage.c
index 2726435e3eda..bc52bc8b59f7 100644
--- a/kernel/bpf/bpf_task_storage.c
+++ b/kernel/bpf/bpf_task_storage.c
@@ -230,17 +230,17 @@ static int bpf_pid_task_storage_delete_elem(struct bpf_map *map, void *key)
/* Called by bpf_task_storage_get*() helpers */
static void *__bpf_task_storage_get(struct bpf_map *map,
struct task_struct *task, void *value,
- u64 flags, gfp_t gfp_flags)
+ u64 flags, gfp_t gfp_flags, bool nobusy)
{
struct bpf_local_storage_data *sdata;

- sdata = task_storage_lookup(task, map, true);
+ sdata = task_storage_lookup(task, map, nobusy);
if (sdata)
return sdata->data;

/* only allocate new storage, when the task is refcounted */
if (refcount_read(&task->usage) &&
- (flags & BPF_LOCAL_STORAGE_GET_F_CREATE)) {
+ (flags & BPF_LOCAL_STORAGE_GET_F_CREATE) && nobusy) {
sdata = bpf_local_storage_update(
task, (struct bpf_local_storage_map *)map, value,
BPF_NOEXIST, gfp_flags);
@@ -254,17 +254,18 @@ static void *__bpf_task_storage_get(struct bpf_map *map,
BPF_CALL_5(bpf_task_storage_get_recur, struct bpf_map *, map, struct task_struct *,
task, void *, value, u64, flags, gfp_t, gfp_flags)
{
+ bool nobusy;
void *data;

WARN_ON_ONCE(!bpf_rcu_lock_held());
if (flags & ~BPF_LOCAL_STORAGE_GET_F_CREATE || !task)
return (unsigned long)NULL;

- if (!bpf_task_storage_trylock())
- return (unsigned long)NULL;
+ nobusy = bpf_task_storage_trylock();
data = __bpf_task_storage_get(map, task, value, flags,
- gfp_flags);
- bpf_task_storage_unlock();
+ gfp_flags, nobusy);
+ if (nobusy)
+ bpf_task_storage_unlock();
return (unsigned long)data;
}

--
2.30.2