Date: Mon, 12 Nov 2001 15:32:21 -0800 (PST)
From: John Baldwin <jhb@FreeBSD.org>
To: Matthew Dillon <dillon@apollo.backplane.com>
Cc: Terry Lambert <tlambert2@mindspring.com>, Robert Watson <rwatson@FreeBSD.ORG>, freebsd-arch@FreeBSD.ORG
Subject: Re: cur{thread/proc}, or not.
Message-ID: <XFMail.011112153221.jhb@FreeBSD.org>
In-Reply-To: <200111122320.fACNKLC07027@apollo.backplane.com>
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On 12-Nov-01 Matthew Dillon wrote: >: >:Yep. They use a mutex for the refcount for now, but I still have patches >:that >:some people don't like for implementing a simple refcount API just using >:atomic >:operations. >: >:-- >: >:John Baldwin <jhb@FreeBSD.org> -- http://www.FreeBSD.org/~jhb/ > > I haven't seen your patches but I like the idea of a simple API for > incrementing and decrementing a refcnt_t type of variable that > hides the underlying 'how'. For example, on some architectures > you could use atomic ops, on others you could use a small pool > of mutexes. http://www.freebsd.org/~jhb/patches/refcount.patch It's slightly different than this in that refcount_drop() returns a boolean that is true if the count just dropped to zero. It only uses mutexes when using debugging and doesn't use a pool, but currently it is implemented completely with atomic ops on all currently supported archs. Hmm, it needs a change in that no memory barriers are really needed except that maybe the atomic_add should use a release barrier. This refcount has some problems, however. The only reliable way to do a refcount_shared() primitive would be to do int refcount_shared(refcount_t *count) { int rval; rval = !refcount_drop(count); refcount_hold(count); } But that is evil and has a race condition. Changing refcount_drop() to return the current value would be more workable I suppose and would allow you to do this by doing a hold and then a drop and see if the value is > 1 to see if it's shared. -- 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/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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