compat_ioctl: simplify lookup table

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The compat_ioctl table now only contains entries for
COMPATIBLE_IOCTL, so we only need to know if a number
is listed in it or now.

As an optimization, we hash the table entries with a
reversible transformation to get a more uniform distribution
over it, sort the table at startup and then guess the
position in the table when an ioctl number gets called
to do a linear search from there.

With the current set of ioctl numbers and the chosen
transformation function, we need an average of four
steps to find if a number is in the set, all of the
accesses within one or two cache lines.

This at least as good as the previous hash table
approach but saves 8.5 kb of kernel memory.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>

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This commit is contained in:

Arnd Bergmann 2009-11-05 19:52:55 +01:00

parent 789f0f8911

commit 661f627da9

1 changed files with 44 additions and 46 deletions

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fs/compat_ioctl.c

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