Date: Tue, 12 Sep 2000 12:31:14 +0930 From: Greg Lehey <grog@lemis.com> To: Joerg Micheel <joerg@cs.waikato.ac.nz> Cc: Matthew Jacob <mjacob@feral.com>, Frank Mayhar <frank@exit.com>, John Baldwin <jhb@pike.osd.bsdi.com>, Mark Murray <markm@freebsd.org>, FreeBSD-arch@freebsd.org Subject: Mutexes and semaphores (was: cvs commit: src/sys/conf files src/sys/sys random.h src/sys/dev/randomdev hash.c hash.h harvest.c randomdev.c yarrow.c yarro) Message-ID: <20000912123114.K88615@wantadilla.lemis.com> In-Reply-To: <20000912145255.A41113@cs.waikato.ac.nz>; from joerg@cs.waikato.ac.nz on Tue, Sep 12, 2000 at 02:52:55PM %2B1200 References: <200009120101.e8C11nN56928@realtime.exit.com> <Pine.LNX.4.21.0009111801490.25916-100000@zeppo.feral.com> <20000912121105.J88615@wantadilla.lemis.com> <20000912145255.A41113@cs.waikato.ac.nz>
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On Tuesday, 12 September 2000 at 14:52:55 +1200, Joerg Micheel wrote: > On Tue, Sep 12, 2000 at 12:11:05PM +0930, Greg Lehey wrote: >> On Monday, 11 September 2000 at 18:02:26 -0700, Matt Jacob wrote: >>>> Greg Lehey wrote: >>>>> I've been wondering whether we shouldn't associate mutexes with data >>>>> structures rather than code. It's possible that it would make it >>>>> easier to avoid deadlocks. Thoughts? >>>> >>>> Speaking as a BSD/OS (and former Unixware) developer: YES! >>> >>> Hmm. I would rather have assumed that this is what mutexes are >>> about. Semaphores gate entry in code. Mutexes provide locking on >>> data. Simple enough. >> >> That's a matter of definition. The big difference I see between a >> semaphore and a blocking "mutex" is that there's no count associated >> with the blocking "mutex": it's a degenerate case of a semaphore. >> >> At Tandem, we used semaphores exclusively (well, we had a mutex >> instruction, but it was really interrupt lockout). As far as I can >> recall, the semaphore counter was always 1, so the effect was >> identical to the current blocking "mutexes". > > I liked the model Sun chose for Solaris. They have mutex', rw_locks, > condition variables. I don't like semaphores. What's the difference between a mutex and a semaphore? > Mutexes are for short locks. Condition variables are for long-term > waits, they are associated with a mutex. You can only sleep/wakeup a > CV when holding the associated with it, which prevents races. When > having to sleep on a CV the kernel would unlock the mutex and > reaquire it for the running thread before returning. Yes, that's pretty much what msleep() does. We're still discussing whether we should have real condition variables. Greg -- Finger grog@lemis.com for PGP public key See complete headers for address and phone numbers To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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