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bpf: Avoid taking spinlock in bpf_task_storage_get if potential deadlock is detected

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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 bc235cdb42 ("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>
Link: https://lore.kernel.org/r/20221025184524.3526117-5-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Martin KaFai Lau
2022-10-25 11:45:19 -07:00
committed by Alexei Starovoitov
parent 6d65500c34
commit e8b02296a6
2 changed files with 9 additions and 7 deletions
Download Patch File Download Diff File Expand all files Collapse all files
kernel/bpf/bpf_local_storage.c
+1
View File
@@ -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,
kernel/bpf/bpf_task_storage.c
+8 -7
View File
@@ -230,17 +230,17 @@ out:
/* 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;
}