Subject: [PATCH v7 bpf-next 0/9] introduce bpf_spin_lock

From: Alexei Starovoitov <[email-masked]>
Date: Thu, 31 Jan 2019 15:40:03 -0800

Many algorithms need to read and modify several variables atomically.
Until now it was hard to impossible to implement such algorithms in BPF.
Hence introduce support for bpf_spin_lock.

The api consists of 'struct bpf_spin_lock' that should be placed
inside hash/array/cgroup_local_storage element
and bpf_spin_lock/unlock() helper function.

Example:
struct hash_elem {
    int cnt;
    struct bpf_spin_lock lock;
};
struct hash_elem * val = bpf_map_lookup_elem(&hash_map, &key);
if (val) {
    bpf_spin_lock(&val->lock);
    val->cnt++;
    bpf_spin_unlock(&val->lock);
}

and BPF_F_LOCK flag for lookup/update bpf syscall commands that
allows user space to read/write map elements under lock.

Together these primitives allow race free access to map elements
from bpf programs and from user space.

Key restriction: root only.
Key requirement: maps must be annotated with BTF.

This concept was discussed at Linux Plumbers Conference 2018.
Thank you everyone who participated and helped to iron out details
of api and implementation.

Patch 1: bpf_spin_lock support in the verifier, BTF, hash, array.
Patch 2: bpf_spin_lock in cgroup local storage.
Patches 3,4,5: tests
Patch 6: BPF_F_LOCK flag to lookup/update
Patches 7,8,9: tests

v6->v7:
- fixed this_cpu->__this_cpu per Peter's suggestion and added Ack.
- simplified bpf_spin_lock and load/store overlap check in the verifier
  as suggested by Andrii
- rebase

v5->v6:
- adopted arch_spinlock approach suggested by Peter
- switched to spin_lock_irqsave equivalent as the simplest way
  to avoid deadlocks in rare case of nested networking progs
  (cgroup-bpf prog in preempt_disable vs clsbpf in softirq sharing
  the same map with bpf_spin_lock)
  bpf_spin_lock is only allowed in networking progs that don't
  have arbitrary entry points unlike tracing progs.
- rebase and split test_verifier tests

v4->v5:
- disallow bpf_spin_lock for tracing progs due to insufficient preemption checks
- socket filter progs cannot use bpf_spin_lock due to missing preempt_disable
- fix atomic_set_release. Spotted by Peter.
- fixed hash_of_maps
  
v3->v4:
- fix BPF_EXIST | BPF_NOEXIST check patch 6. Spotted by Jakub. Thanks!
- rebase

v2->v3:
- fixed build on ia64 and archs where qspinlock is not supported
- fixed missing lock init during lookup w/o BPF_F_LOCK. Spotted by Martin

v1->v2:
- addressed several issues spotted by Daniel and Martin in patch 1
- added test11 to patch 4 as suggested by Daniel

Alexei Starovoitov (9):
  bpf: introduce bpf_spin_lock
  bpf: add support for bpf_spin_lock to cgroup local storage
  tools/bpf: sync include/uapi/linux/bpf.h
  selftests/bpf: add bpf_spin_lock verifier tests
  selftests/bpf: add bpf_spin_lock C test
  bpf: introduce BPF_F_LOCK flag
  tools/bpf: sync uapi/bpf.h
  libbpf: introduce bpf_map_lookup_elem_flags()
  selftests/bpf: test for BPF_F_LOCK

 include/linux/bpf.h                           |  39 ++-
 include/linux/bpf_verifier.h                  |   1 +
 include/linux/btf.h                           |   1 +
 include/uapi/linux/bpf.h                      |   8 +-
 kernel/Kconfig.locks                          |   3 +
 kernel/bpf/arraymap.c                         |  23 +-
 kernel/bpf/btf.c                              |  42 +++
 kernel/bpf/core.c                             |   2 +
 kernel/bpf/hashtab.c                          |  63 +++-
 kernel/bpf/helpers.c                          |  96 +++++
 kernel/bpf/local_storage.c                    |  16 +-
 kernel/bpf/map_in_map.c                       |   5 +
 kernel/bpf/syscall.c                          |  45 ++-
 kernel/bpf/verifier.c                         | 171 ++++++++-
 net/core/filter.c                             |  16 +-
 tools/include/uapi/linux/bpf.h                |   8 +-
 tools/lib/bpf/bpf.c                           |  13 +
 tools/lib/bpf/bpf.h                           |   2 +
 tools/lib/bpf/libbpf.map                      |   1 +
 tools/testing/selftests/bpf/Makefile          |   2 +-
 tools/testing/selftests/bpf/bpf_helpers.h     |   4 +
 tools/testing/selftests/bpf/test_map_lock.c   |  66 ++++
 tools/testing/selftests/bpf/test_progs.c      | 117 ++++++-
 tools/testing/selftests/bpf/test_spin_lock.c  | 108 ++++++
 tools/testing/selftests/bpf/test_verifier.c   | 104 +++++-
 .../selftests/bpf/verifier/spin_lock.c        | 331 ++++++++++++++++++
 26 files changed, 1248 insertions(+), 39 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/test_map_lock.c
 create mode 100644 tools/testing/selftests/bpf/test_spin_lock.c
 create mode 100644 tools/testing/selftests/bpf/verifier/spin_lock.c

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2.20.0