Date: Mon, 25 Sep 2000 13:24:54 -0700 (PDT) From: John Polstra <jdp@polstra.com> To: arch@freebsd.org Cc: tlambert@primenet.com Subject: Re: Mutexes and semaphores (was: cvs commit: src/sys/conf files Message-ID: <200009252024.NAA02690@vashon.polstra.com> In-Reply-To: <200009252006.NAA00200@usr02.primenet.com> References: <200009252006.NAA00200@usr02.primenet.com>
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In article <200009252006.NAA00200@usr02.primenet.com>, Terry Lambert <tlambert@primenet.com> wrote: > > You are wrong. Counting semaphores do not keep track of owners. > > OK. Let's be pedantic. Neither do mutexes. I didn't say "mutex", I said "recursive mutex". Recursive mutexes do indeed keep track of their owners. > Counting semaphores are a more appropriate primitive, as the > "resource" which is counted is the ownership capability. As > others have pointed out (Archie, etc.), a semaphore with a > count of 1 is appropriate. When the count goes 1->0, then > we can consider that ownership has been relinquished. Actually, when the count goes 1->0, ownership has been acquired, not relinquished. The count represents the number of available units, and that is the case in every definition and every implementation of semaphores I have ever seen (which is quite a few, beginning in the early 70's.). It's even true in the rather baroque implementation of semop(3). > I never stated that the recursion count would be implemented in > the semaphore count of a counting semaphore. Please read the > first quoted sentence again. Ownership and recursion are kept > in the seperate struct. Fine, then you don't need a counting semaphore at all, as a simple non-recursive mutex will do the same job just as well and more efficiently. > To get around this, you have to implement non-recusing mutexes > using a semaphore of count 1. A semaphore with a count of 1, when used for mutual exclusion, behaves exactly the same as a simple mutex. I don't understand why you brought up counting semaphores at all. John -- John Polstra jdp@polstra.com John D. Polstra & Co., Inc. Seattle, Washington USA "Disappointment is a good sign of basic intelligence." -- Chögyam Trungpa To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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