Re: [Lse-tech] [PATCH] new bitmap list format (for cpusets)

[Paul Jackson]

Martin wrote:
> OK, that [removal of prefix char fluff^Wstuff] looks a lot more palletable ;-)

Good.


> it looks like you're only parsing on the read-side to me

I couldn't parse this comment, but hopefully I'll answer this below ...


> I can't see any [bitmap_scnlistprintf] callers,

The first caller of the bitmap list format stuff is the next patch - the
cpuset patch.

The bitmap list printing call stack for cpumasks is:

	kernel/cpuset.c:		cpuset_common_file_read()
	kernel/cpuset.c:		cpuset_sprintf_cpulist()
	include/linux/cpumask.h:	__cpulist_scnprintf()
	lib/bitmap.c:			bitmap_scnlistprintf()

Similarly for nodemasks.

That's why the very first two lines of the bitmap list patch comment
state:

	A bitmap print and parse format that provides lists of ranges of
	numbers, to be first used for by cpusets (next patch).

;).

> do we really need both cpumask_parse and __cpumask_parse in front of
> bitmap_parse?

Yes.  Look through cpumask.h and nodemask.h.  You will see this use of
a #define macro wrapping a static inline macro repeatedly.  The #define
macro is needed to implement the implied call-by-reference convention,
and the static inline is needed to add some type checking.

Note that while the C syntax for using these cpu and node mask operators
_looks_ like it is passing the mask by value at the point marked "<==",
which would copy a possibly multiple word mask on the stack, in the
code:

	static int cpuset_sprintf_cpulist(char *page, struct cpuset *cs)
	{
        	cpumask_t mask;
		...
        	return cpulist_scnprintf(page, PAGE_SIZE, mask);	/* <== */
	}

this code is _really_ passing a pointer (unsigned long *) on the stack
to the actual implementing code, in lib/bitmap.c:bitmap_scnlistprintf().

The key ingredient that the #define macro adds is the '&' char, turning
normal C call-by-value conventions into an implied call-by-reference.

This somewhat un-C-like calling convention (more like Pascal call by
reference) existed for cpumasks before I came on the scene, with my
various cleanups of the bitmap and cpumask code over the last 10 months.
I found the convention pleasing enough, if odd, so I preserved it, for
most of the cpumask calls, and now (thanks to Matthew Dobson) the
nodemask calls as well.

In summary, the #define macros are needed in cpumask.h and nodemask.h to
alter the usual C call-by-value conventions, and the static inline
macros are needed to provide some static type checking of the arguments.

... at least I hope that was your question.

-- 
                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <pj@xxxxxxx> 1.650.933.1373
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