locking/atomic: scripts: generate kerneldoc comments

Currently the atomics are documented in Documentation/atomic_t.txt, and
have no kerneldoc comments. There are a sufficient number of gotchas
(e.g. semantics, noinstr-safety) that it would be nice to have comments
to call these out, and it would be nice to have kerneldoc comments such
that these can be collated.

While it's possible to derive the semantics from the code, this can be
painful given the amount of indirection we currently have (e.g. fallback
paths), and it's easy to be mislead by naming, e.g.

* The unconditional void-returning ops *only* have relaxed variants
without a _relaxed suffix, and can easily be mistaken for being fully
ordered.

It would be nice to give these a _relaxed() suffix, but this would
result in significant churn throughout the kernel.

* Our naming of conditional and unconditional+test ops is rather
inconsistent, and it can be difficult to derive the name of an
operation, or to identify where an op is conditional or
unconditional+test.

Some ops are clearly conditional:
- dec_if_positive
- add_unless
- dec_unless_positive
- inc_unless_negative

Some ops are clearly unconditional+test:
- sub_and_test
- dec_and_test
- inc_and_test

However, what exactly those test is not obvious. A _test_zero suffix
might be clearer.

Others could be read ambiguously:
- inc_not_zero // conditional
- add_negative // unconditional+test

It would probably be worth renaming these, e.g. to inc_unless_zero and
add_test_negative.

As a step towards making this more consistent and easier to understand,
this patch adds kerneldoc comments for all generated *atomic*_*()
functions. These are generated from templates, with some common text
shared, making it easy to extend these in future if necessary.

I've tried to make these as consistent and clear as possible, and I've
deliberately ensured:

* All ops have their ordering explicitly mentioned in the short and long
description.

* All test ops have "test" in their short description.

* All ops are described as an expression using their usual C operator.
For example:

andnot: "Atomically updates @v to (@v & ~@i)"
inc: "Atomically updates @v to (@v + 1)"

Which may be clearer to non-naative English speakers, and allows all
the operations to be described in the same style.

* All conditional ops have their condition described as an expression
using the usual C operators. For example:

add_unless: "If (@v != @u), atomically updates @v to (@v + @i)"
cmpxchg: "If (@v == @old), atomically updates @v to @new"

Which may be clearer to non-naative English speakers, and allows all
the operations to be described in the same style.

* All bitwise ops (and,andnot,or,xor) explicitly mention that they are
bitwise in their short description, so that they are not mistaken for
performing their logical equivalents.

* The noinstr safety of each op is explicitly described, with a
description of whether or not to use the raw_ form of the op.

There should be no functional change as a result of this patch.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230605070124.3741859-26-mark.rutland@arm.com