This reverts commit e463b6de9da1 ("bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()"). The original commit removed the KF_ACQUIRE flag from bpf_get_root_mem_cgroup() under the assumption that it resulted in simplified usage. This stemmed from the fact that bpf_get_root_mem_cgroup() inherently returns a reference to an object which technically isn't reference counted, therefore there is no strong requirement to call a matching bpf_put_mem_cgroup() on the returned reference. Although technically correct, as per the arguments in the thread [0], dropping the KF_ACQUIRE flag and losing reference tracking semantics negatively impacted the usability of bpf_get_root_mem_cgroup() in practice. [0] https://lore.kernel.org/bpf/878qdx6yut.fsf@linux.dev/ Link: https://lore.kernel.org/bpf/CAADnVQ+6d1Lj4dteAv8u62d7kj3Ze5io6bqM0xeQd-UPk9ZgJQ@mail.gmail.com/ Signed-off-by: Matt Bobrowski --- mm/bpf_memcontrol.c | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-) diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c index f95cd5d16f4c..716df49d7647 100644 --- a/mm/bpf_memcontrol.c +++ b/mm/bpf_memcontrol.c @@ -13,12 +13,12 @@ __bpf_kfunc_start_defs(); /** * bpf_get_root_mem_cgroup - Returns a pointer to the root memory cgroup * - * Return: A pointer to the root memory cgroup. Notably, the pointer to the - * returned struct mem_cgroup is trusted by default, so it's perfectably - * acceptable to also pass this pointer into other BPF kfuncs (e.g., - * bpf_mem_cgroup_usage()). Additionally, this BPF kfunc does not make use of - * KF_ACQUIRE semantics, so there's no requirement for the BPF program to call - * bpf_put_mem_cgroup() on the pointer returned by this BPF kfunc. + * The function has KF_ACQUIRE semantics, even though the root memory + * cgroup is never destroyed after being created and doesn't require + * reference counting. And it's perfectly safe to pass it to + * bpf_put_mem_cgroup() + * + * Return: A pointer to the root memory cgroup. */ __bpf_kfunc struct mem_cgroup *bpf_get_root_mem_cgroup(void) { @@ -162,7 +162,7 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(struct mem_cgroup *memcg) __bpf_kfunc_end_defs(); BTF_KFUNCS_START(bpf_memcontrol_kfuncs) -BTF_ID_FLAGS(func, bpf_get_root_mem_cgroup, KF_RET_NULL) +BTF_ID_FLAGS(func, bpf_get_root_mem_cgroup, KF_ACQUIRE | KF_RET_NULL) BTF_ID_FLAGS(func, bpf_get_mem_cgroup, KF_ACQUIRE | KF_RET_NULL | KF_RCU) BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE) -- 2.52.0.457.g6b5491de43-goog