Date: Tue, 12 Dec 2000 21:50:19 -0500 (EST) From: Garrett Wollman <wollman@khavrinen.lcs.mit.edu> To: Alfred Perlstein <bright@wintelcom.net> Cc: net@FreeBSD.ORG Subject: Re: MEXT_IS_REF broken. Message-ID: <200012130250.VAA55319@khavrinen.lcs.mit.edu> In-Reply-To: <20001212175937.M16205@fw.wintelcom.net> References: <20001211014837.W16205@fw.wintelcom.net> <Pine.BSF.4.21.0012111223350.21769-100000@jehovah.technokratis.com> <20001212014429.Y16205@fw.wintelcom.net> <20001212015059.Z16205@fw.wintelcom.net> <20001212143214.H2312@canonware.com> <20001212175937.M16205@fw.wintelcom.net>
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<<On Tue, 12 Dec 2000 17:59:37 -0800, Alfred Perlstein <bright@wintelcom.net> said:
> Actually, in truth I think you can get the code right like so:
> long x = _mmm->m_ext.ref_cnt->refcnt;
> while (!atomic_cmpset_long(&_mmm->m_ext.ref_cnt->refcnt, x - 1, x))
> ;
Cool! You've just (almost) reinvented non-blocking parallel
reference-counts. Of course, what you really need is:
long
atomic_decrement_long(long *where)
{
long oldval;
do {
oldval = *where;
} while (compare_exchange(where, &oldval, oldval - 1) != FAILURE);
return (oldval);
/*
* Five instructions in-line on i486.
* 1: movl (%ebx), %eax
* movl %eax, %edx
* subl $1, %edx
* cmpxchg (%ebx), %eax, %edx ; IIRC -- might be backwards
* jc 1
*/
}
...except that on some architectures, the right way to write it would
be:
long
atomic_decrement_long(long *where)
{
long oldval;
do {
oldval = load_linked_long(where);
} while (store_conditional(where, oldval - 1) != FAILURE);
return (oldval);
/*
* Compiles to four or five instructions on an Alpha.
*/
}
In this particular instance, you know that you just deleted the last
reference if atomic_decrement_long returns an `old value' of
> But that's just gross, expensive and shouldn't be needed.
Nothing gross about it -- just ask any parallel algorithms geek. (Of
which I am emphatically not one, I should point out.)
-GAWollman
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