atomic_fetchadd_int() and a simple refcount API for non-complicatedrefcounts

[Andre Oppermann]

John Baldwin wrote:
> 
> I have two patches here for review.  They have been tested on i386, alpha, and
> sparc64 so far.  The first patch adds a new atomic operation:
> atomic_fetchadd_int() (and its alias: atomic_fetchadd_32()).  The operation
> pretty much maps directly onto the x86 xadd instruction added with the 486.
> I believe it also maps directly onto the fetchadd instruction on ia64 (from
> whence it got its name).  (Note that the ia64 instruction can only take a
> fixed set of immediate operands to add by such as 1, -1, 2, -2, 4, -4, etc.
> up to 64 and -64 IIRC.)  On architectures where I wasn't sure how to do the
> inline assembly (ia64, ppc, arm) it just uses a atomic_cmpset() loop, but
> that can be changed later as an optimization easily enough.  Having this new
> primitive allows for construction of simple standalone reference counts that
> are cheaper than ones protected by a mutex (1 atomic op vs at least 2).
> Thus, I used atomic_fetchadd() to build a very simple refcount API that
> operates on integers.  It has an init method to set the initial value, an
> acquire method to bump a refcount, and a release method to drop a reference.
> The release method returns non-zero when the last reference is dropped.  I
> used this API to implement the reference counts for ucreds, plimits, pargs,
> and mbuf clusters (though I know andre@ has plans in the works to hack on the
> mbuf ones further, so those changes might just be temporary).  Also, I know
> that andre@ wants atomic_fetchadd() for the changes he will be doing.
> Patches are at the URLS below:
> 
> http://www.FreeBSD.org/~jhb/patches/atomic_fetchadd.patch
> http://www.FreeBSD.org/~jhb/patches/refcount_cvs.patch

I have the i386 and amd64 versions of atomic_fetchadd running in my tree
for about a week.  Works fine so far.  ;-)

Please don't convert the mbuf cluster refcount to the refcount API.  I
can do my commit reworking mbuf ref counting right after you commit adding
the new atomic_fetchadd function.  That way we can save one step and
I have less trouble re-integrating my tree with HEAD.

-- 
Andre


> FAQ:
> 
> Q: The refcount API is too simple and doesn't handle the snortzel-foo
>    edge case!!!!
> 
> A: Yes.  It's not meant to be all-singing and all-dancing.  It is simply
>    available as one tool that is available for use.  More complex reference
>    counting schemes can be built using atomic_fetchadd() or other more complex
>    primitives such as mutexes if needed.
> 
> Q: I think the name 'fetchadd' sucks!!
> 
> A: Your opinion has been noted.
> 
> Q: Will this destroy the ABI?
> 
> A: No.  The refcount_*() functions are all inlines.  The only thing that might
>    break the ABI might be changes to structures to remove mutexes or mutex
>    pointers, but that part of a change can always be left out if this were
>    merged across a branch.
> 
> Q: Will this be in 6.0?
> 
> A: Hopefully.  The sooner this gets reviewed the better the chances.  Due to
>    the previous question, it is possible to merge it to RELENG_6 after 6.0 is
>    released anyway.
> 
> Q: What color is a snortzel-foo?
> 
> A: I don't know, probably some sort of lavender.
> 
> --
> John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
> "Power Users Use the Power to Serve"  =  http://www.FreeBSD.org
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