From: Martin KaFai Lau martin.lau@kernel.org
[ Upstream commit f4edc66e48a694b3e6d164cc71f059de542dfaec ]
The current cgrp storage has a percpu counter, bpf_cgrp_storage_busy, to detect potential deadlock at a spin_lock that the local storage acquires during new storage creation.
There are false positives. It turns out to be too noisy in production. For example, a bpf prog may be doing a bpf_cgrp_storage_get on map_a. An IRQ comes in and triggers another bpf_cgrp_storage_get on a different map_b. It will then trigger the false positive deadlock check in the percpu counter. On top of that, both are doing lookup only and no need to create new storage, so practically it does not need to acquire the spin_lock.
The bpf_task_storage_get already has a strategy to minimize this false positive by only failing if the bpf_task_storage_get needs to create a new storage and the percpu counter is busy. Creating a new storage is the only time it must acquire the spin_lock.
This patch borrows the same idea. Unlike task storage that has a separate variant for tracing (_recur) and non-tracing, this patch stays with one bpf_cgrp_storage_get helper to keep it simple for now in light of the upcoming res_spin_lock.
The variable could potentially use a better name noTbusy instead of nobusy. This patch follows the same naming in bpf_task_storage_get for now.
I have tested it by temporarily adding noinline to the cgroup_storage_lookup(), traced it by fentry, and the fentry program succeeded in calling bpf_cgrp_storage_get().
Signed-off-by: Martin KaFai Lau martin.lau@kernel.org Link: https://lore.kernel.org/r/20250318182759.3676094-1-martin.lau@linux.dev Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/bpf_cgrp_storage.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/kernel/bpf/bpf_cgrp_storage.c b/kernel/bpf/bpf_cgrp_storage.c index 7996fcea3755e..c535e8f46d2cd 100644 --- a/kernel/bpf/bpf_cgrp_storage.c +++ b/kernel/bpf/bpf_cgrp_storage.c @@ -162,6 +162,7 @@ BPF_CALL_5(bpf_cgrp_storage_get, struct bpf_map *, map, struct cgroup *, cgroup, void *, value, u64, flags, gfp_t, gfp_flags) { struct bpf_local_storage_data *sdata; + bool nobusy;
WARN_ON_ONCE(!bpf_rcu_lock_held()); if (flags & ~(BPF_LOCAL_STORAGE_GET_F_CREATE)) @@ -170,21 +171,21 @@ BPF_CALL_5(bpf_cgrp_storage_get, struct bpf_map *, map, struct cgroup *, cgroup, if (!cgroup) return (unsigned long)NULL;
- if (!bpf_cgrp_storage_trylock()) - return (unsigned long)NULL; + nobusy = bpf_cgrp_storage_trylock();
- sdata = cgroup_storage_lookup(cgroup, map, true); + sdata = cgroup_storage_lookup(cgroup, map, nobusy); if (sdata) goto unlock;
/* only allocate new storage, when the cgroup is refcounted */ if (!percpu_ref_is_dying(&cgroup->self.refcnt) && - (flags & BPF_LOCAL_STORAGE_GET_F_CREATE)) + (flags & BPF_LOCAL_STORAGE_GET_F_CREATE) && nobusy) sdata = bpf_local_storage_update(cgroup, (struct bpf_local_storage_map *)map, value, BPF_NOEXIST, false, gfp_flags);
unlock: - bpf_cgrp_storage_unlock(); + if (nobusy) + bpf_cgrp_storage_unlock(); return IS_ERR_OR_NULL(sdata) ? (unsigned long)NULL : (unsigned long)sdata->data; }