Date: Tue, 31 Oct 2000 11:15:14 -0800 (PST) From: John Baldwin <jhb@FreeBSD.org> To: Alfred Perlstein <bright@wintelcom.net> Cc: freebsd-smp@FreeBSD.org, Robert Watson <rwatson@FreeBSD.org>, Cedric Berger <cedric@wireless-networks.com> Subject: Re: Reference count invariants in a fine-grained threaded enviro Message-ID: <XFMail.001031111514.jhb@FreeBSD.org> In-Reply-To: <20001031102110.V22110@fw.wintelcom.net>
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On 31-Oct-00 Alfred Perlstein wrote:
> * Cedric Berger <cedric@wireless-networks.com> [001031 10:11] wrote:
>>
>>
>> Robert Watson wrote:
>> >
>> [...]
>> >
>> > Rules for interactions between mutexes and reference-counted kernel
>> > objects:
>> >
>> > Assumptions:
>> >
>> > - Objects with reference counts have a mutex that can protect their
>> > reference count, and possibly other variables (or other instances). For
>> > example, struct cred might have a mutex per instance, but all struct
>> > prison's might use the same mutex.
>> >
>>
>> Is there not a cheaper way to manage reference count then using mutexes?
>> I would guess that all architectures must have hardware support (i.e.
>> special
>> assembly instruction) to atomically increment/decrement+read a 32-bit value
>> in a multiprocessor environment
>
> Yes, but FreeBSD is has too many knights that say NIH!
Huh? /me confused
> This is where atomic refcounts would simplify the code however for some
> reason everyone I've talked to prefers to have this sort of code:
Really? Who have you been talking to? :) You can already do this now
like so (the KTR code does this for example):
static inline int
refcount_update(int *ref, int delta)
{
int old;
do {
old = *ref;
} while (!atomic_cmpset(ref, old, old + delta);
return (old + delta);
}
Then just do:
{
...
/* add a reference */
refcount_update(&foo->refcount, 1);
...
/* remove a reference */
refcount_update(&foo->refcount, -1);
}
--
John Baldwin <jhb@FreeBSD.org> -- http://www.FreeBSD.org/~jhb/
PGP Key: http://www.baldwin.cx/~john/pgpkey.asc
"Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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