[RFC 0/3] headers: start rework to avoid recursive inclusion

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

I've gotten back to a patch series I worked on a while back, to clean
up the way we include headers recursively in the kernel, analysing the
dependency chains and reducing them as much as possible. The most common
issue is headers that need a data structure definition from another
header but shouldn't really pull in the rest of it.

Improving this will help both the compilation speed of single files
(earlier experiments with clang showed a crazy 30% improvement
after reducing the average size of the indirect headers by 90%),
and those people that build a lot of similar kernels, where a
modification in a common header tends to require rebuilding
everything.

These three patches are a first step in the direction of minimizing
the headers needed for defining data structures that embed other
structures. Rather than split up each header into one that only defines
structures, and another one for the rest, this adds more fundamental
types to linux/types.h and introduces a new linux/struct_types.h with
the most commonly embedded structures.

The fs_types.h change in the third patch shows how high-level structure
definitions can then be done with a much smaller set of indirect includes,
using one of the central headers (linux/fs.h) as the example.  The same
would need to be done for other headers that are currently included in
most drivers and that in turn indirectly include hundreds of other headers
(linux/skbuff.h, linux/mm.h, linux/device.h, linux/sched.h, ...).

None of this work actually removes the indirect includes yet, as that
can only be done after each .c file that currently relies on indirect
includes of common headers is changed to include them directly.
The first step toward that however is to come to a rough agreement
on what the structure should be.

Any comments, suggestions, ideas?

       Arnd

Arnd Bergmann (3):
  headers: add more types to linux/types.h
  headers: introduce linux/struct_types.h
  headers: repurpose linux/fs_types.h

 arch/alpha/include/asm/spinlock_types.h       |    2 +-
 arch/arc/include/asm/atomic64-arcv2.h         |    4 -
 arch/arm/include/asm/atomic.h                 |    4 -
 arch/arm/include/asm/spinlock_types.h         |    2 +-
 arch/arm64/include/asm/spinlock_types.h       |    2 +-
 arch/csky/include/asm/spinlock_types.h        |    2 +-
 arch/hexagon/include/asm/spinlock_types.h     |    2 +-
 arch/ia64/include/asm/spinlock_types.h        |    2 +-
 .../include/asm/simple_spinlock_types.h       |    2 +-
 arch/powerpc/include/asm/spinlock_types.h     |    2 +-
 arch/riscv/include/asm/spinlock_types.h       |    2 +-
 arch/s390/include/asm/spinlock_types.h        |    2 +-
 arch/sh/include/asm/spinlock_types.h          |    2 +-
 arch/x86/include/asm/atomic64_32.h            |    4 -
 arch/xtensa/include/asm/spinlock_types.h      |    2 +-
 include/asm-generic/atomic64.h                |    4 -
 include/linux/atomic/atomic-long.h            |    4 +-
 include/linux/bitops.h                        |    6 -
 include/linux/bits.h                          |    6 +
 include/linux/bvec.h                          |   18 -
 include/linux/completion.h                    |   17 -
 include/linux/cpumask.h                       |    3 -
 include/linux/fs.h                            | 1151 +---------------
 include/linux/fs_types.h                      | 1225 ++++++++++++++++-
 include/linux/hrtimer.h                       |   32 -
 include/linux/kobject.h                       |   18 -
 include/linux/kref.h                          |    4 -
 include/linux/ktime.h                         |    3 -
 include/linux/list_bl.h                       |    7 -
 include/linux/list_lru.h                      |   15 -
 include/linux/llist.h                         |    8 -
 include/linux/mmzone.h                        |    3 -
 include/linux/mutex.h                         |   51 -
 include/linux/osq_lock.h                      |    8 -
 include/linux/percpu-rwsem.h                  |   11 -
 include/linux/pid.h                           |    9 -
 include/linux/plist.h                         |   10 -
 include/linux/quota.h                         |   29 -
 include/linux/rcu_sync.h                      |    9 -
 include/linux/rcuwait.h                       |   12 -
 include/linux/refcount.h                      |   12 -
 include/linux/rtmutex.h                       |    8 +-
 include/linux/rwbase_rt.h                     |    5 -
 include/linux/rwlock_types.h                  |   19 -
 include/linux/rwsem.h                         |   40 -
 include/linux/seqlock.h                       |   31 -
 include/linux/spinlock_types.h                |   21 -
 include/linux/spinlock_types_raw.h            |   21 +-
 include/linux/spinlock_types_up.h             |    2 +-
 include/linux/struct_types.h                  |  483 +++++++
 include/linux/swait.h                         |   12 -
 include/linux/time64.h                        |   13 -
 include/linux/timer.h                         |   16 +-
 include/linux/timerqueue.h                    |   12 +-
 include/linux/types.h                         |   90 +-
 include/linux/uidgid.h                        |    9 -
 include/linux/uuid.h                          |    6 -
 include/linux/wait.h                          |   29 -
 include/linux/workqueue.h                     |   27 -
 include/linux/xarray.h                        |   23 -
 60 files changed, 1822 insertions(+), 1756 deletions(-)
 create mode 100644 include/linux/struct_types.h

-- 
2.29.2

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: kernel test robot <lkp@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: kernelci@groups.io
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kbuild@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: llvm@lists.linux.dev