Merge branch 'Use this_cpu_xxx for preemption-safety'

Hou Tao says:

====================

From: Hou Tao <houtao1@huawei.com>

Hi,

The patchset aims to make the update of per-cpu prog->active and per-cpu
bpf_task_storage_busy being preemption-safe. The problem is on same
architectures (e.g. arm64), __this_cpu_{inc|dec|inc_return} are neither
preemption-safe nor IRQ-safe, so under fully preemptible kernel the
concurrent updates on these per-cpu variables may be interleaved and the
final values of these variables may be not zero.

Patch 1 & 2 use the preemption-safe per-cpu helpers to manipulate
prog->active and bpf_task_storage_busy. Patch 3 & 4 add a test case in
map_tests to show the concurrent updates on the per-cpu
bpf_task_storage_busy by using __this_cpu_{inc|dec} are not atomic.

Comments are always welcome.

Regards,
Tao

Change Log:
v2:
* Patch 1: update commit message to indicate the problem is only
  possible for fully preemptible kernel
* Patch 2: a new patch which fixes the problem for prog->active
* Patch 3 & 4: move it to test_maps and make it depend on CONFIG_PREEMPT

v1: https://lore.kernel.org/bpf/20220829142752.330094-1-houtao@huaweicloud.com/
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>