[lkml]   [2012]   [Feb]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Subject[PATCH 0/3] Eliminating __FD_*() functions from the kernel

Having examined H. Peter Anvin's patches to merge asm/posix_types.h together, I
think that we can go further: The FD_SET(), FD_CLR() and FD_ISSET() macros are
only used within the kernel itself in relation to the arrays of fd-is-open
flags and close-on-exec flags. FD_ZERO() is not used at all within the kernel.

Now, the FD_SET() & co. wrappers must be exported to userspace, so they must be
kept, but the __FD_SET() & co. implementations behind them on most arches are
_not_ so exported. Possibly PowerPC, MN10300 and Xtensa would survive their
removal entirely from the userspace API as per Peter's patches.

Examining the two remaining usages of struct fd_set, and FD_*() and __FD_*()
for the arrays of bits in struct fdtable, it would seem that we could just move
to using unsigned long directly rather than fd_set, and using __set_bit(),
__clear_bit() and test_bit() upon those.

This would also get rid of two abuses of the fd_set struct within this code:

(1) Most of the time, the two fd_sets pointed to by struct fdtable are not the
length they're supposed to be, and are, in fact, allocated short. This
means FD_ZERO() may not be used on them, for example.

Basically, in effect, alloc_fdmem() allocates an array of unsigned longs
just long enough and then casts it to an fd_set - so why bother with the
fd_set at all?

(2) Non-core VFS code (such as SELinux) sometimes wants to look through the
array of unsigned longs inside the fd_set for the purposes of more
efficient iteration over the entire set of open fds - so why not just
eliminate the fd_set and go directly with an array of unsigned longs?

Furthermore, within the kernel, __FD_SET() and co. are redundant bitops and,
in the original implementation, may actually be less efficient. For instance,
on x86/x86_64, __set_bit() uses the BTS instruction, but __FD_SET() may well
not (depending on the optimiser). I note that Peter's patch does alter this.

So my patches do the following:

(1) Wrap the normal set/bit/test operations on the open_fds and close_on_exec
fd_sets so that the access to them is, for the most part, not open coded.

There's still the issue of the things that poke around inside the fd_sets
behind the scenes (SELinux for example).

(2) Replace the fd_set with an unsigned long array and use __set_bit() and
co. instead of FD_SET() and co.

It might just be better to replace the uses of FD_SET() and co. with
__set_bit() and co. directly, and not bother with wrappers, but the
wrappers are more greppable.

(3) Delete __FD_SET() and co. from linux/time.h.

And are implemented to go on top of Peter's posix_types patchset.

David Howells (3):
Delete the __FD_*() funcs for operating on fd_set from linux/time.h
Replace the fd_sets in struct fdtable with an array of unsigned longs
Wrap accesses to the fd_sets in struct fdtable

Documentation/filesystems/files.txt | 4 +-
arch/powerpc/platforms/cell/spufs/coredump.c | 2 -
drivers/staging/android/binder.c | 10 ++---
fs/autofs4/dev-ioctl.c | 2 -
fs/exec.c | 8 ++--
fs/fcntl.c | 18 ++++-----
fs/file.c | 54 +++++++++++++-------------
fs/open.c | 4 +-
fs/proc/base.c | 2 -
fs/select.c | 2 -
include/linux/fdtable.h | 46 ++++++++++++++++------
include/linux/time.h | 22 -----------
kernel/exit.c | 2 -
security/selinux/hooks.c | 2 -
14 files changed, 88 insertions(+), 90 deletions(-)

 \ /
  Last update: 2012-02-16 19:19    [W:0.189 / U:7.784 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site