bpf: Implement prealloc for task_local_storage

task_local_storage currently does not support pre-allocation and the memory
is allocated on demand using the GFP_ATOMIC mask. While atomic allocations
succeed most of the time and the occasional failures aren't a problem for
many use-cases, there are some which can benefit from reliable allocations -
e.g. tracking acquisitions and releases of specific resources to root cause
long-term reference leaks.

This patchset implements prealloc support for task_local_storage so that
once a map is created, it's guaranteed that the storage area is always
available for all tasks unless explicitly deleted.

The only tricky part is syncronizing against the fork path. Fortunately,
cgroup needs to do the same thing and is already using
cgroup_threadgroup_rwsem and we can use the same mechanism without adding
extra overhead. This patchset generalizes the rwsem and make cgroup and bpf
select it.

This patchset is on top of bpf-next 223f903e9c83 ("bpf: Rename BTF_KIND_TAG
to BTF_KIND_DECL_TAG") and contains the following patches:

 0001-cgroup-Drop-cgroup_-prefix-from-cgroup_threadgroup_r.patch
 0002-sched-cgroup-Generalize-threadgroup_rwsem.patch
 0003-bpf-Implement-prealloc-for-task_local_storage.patch

and also available in the following git branch:

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git bpf-task-local-storage-prealloc

diffstat follows. Thanks.

 fs/exec.c                                                   |    7 +
 include/linux/bpf.h                                         |    6 +
 include/linux/bpf_local_storage.h                           |   12 +++
 include/linux/cgroup-defs.h                                 |   33 ---------
 include/linux/cgroup.h                                      |   11 +--
 include/linux/sched/threadgroup_rwsem.h                     |   46 ++++++++++++
 init/Kconfig                                                |    4 +
 kernel/bpf/Kconfig                                          |    1 
 kernel/bpf/bpf_local_storage.c                              |  112 ++++++++++++++++++++++--------
 kernel/bpf/bpf_task_storage.c                               |  138 +++++++++++++++++++++++++++++++++++++-
 kernel/cgroup/cgroup-internal.h                             |    4 -
 kernel/cgroup/cgroup-v1.c                                   |    9 +-
 kernel/cgroup/cgroup.c                                      |   74 ++++++++++++--------
 kernel/cgroup/pids.c                                        |    2 
 kernel/fork.c                                               |   16 ++++
 kernel/sched/core.c                                         |    4 +
 kernel/sched/sched.h                                        |    1 
 kernel/signal.c                                             |    7 +
 tools/testing/selftests/bpf/prog_tests/task_local_storage.c |  101 +++++++++++++++++++++++++++
 tools/testing/selftests/bpf/progs/task_ls_prealloc.c        |   15 ++++
 20 files changed, 489 insertions(+), 114 deletions(-)

--
tejun