From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Sat, 17 Dec 2005 17:21:32 -0700 From: Matthew Wilcox Subject: Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation Message-ID: <20051218002132.GI2361@parisc-linux.org> References: <1134770778.2806.31.camel@tglx.tec.linutronix.de> <1134772964.2806.50.camel@tglx.tec.linutronix.de> <20051217002929.GA7151@tsunami.ccur.com> <20051217234305.GH2361@parisc-linux.org> <1134864321.11227.52.camel@mindpipe> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1134864321.11227.52.camel@mindpipe> To: Lee Revell Cc: Linus Torvalds , Steven Rostedt , Joe Korty , Thomas Gleixner , Geert Uytterhoeven , Andrew Morton , linux-arch@vger.kernel.org, Linux Kernel Development , arjan@infradead.org, Christoph Hellwig , mingo@elte.hu, Alan Cox , nikita@clusterfs.com, pj@sgi.com, dhowells@redhat.com List-ID: On Sat, Dec 17, 2005 at 07:05:21PM -0500, Lee Revell wrote: > On Sat, 2005-12-17 at 16:43 -0700, Matthew Wilcox wrote: > > I have a better example of something we currently get wrong that I > > haven't heard any RT person worry about yet. If two tasks are sleeping > > on the same semaphore, the one to be woken up will be the first one to > > wait for it, not the highest-priority task. > > > > Obviously, this was introduced by the wake-one semantics. But how to > > fix it? Should we scan the entire queue looking for the best task to > > wake? Should we try to maintain the wait list in priority order? Or > > should we just not care? Should we document that we don't care? ;-) > > It's well known that this is a problem: > > http://developer.osdl.org/dev/robustmutexes/src/fusyn.hg/Documentation/fusyn/fusyn-why.txt Erm. That paper is talking about user-space semaphores based on futexes. I'm talking about kernel semaphores. At a first glance, fixing futexes would be a very different job from fixing semaphores. BTW, fuqueues? HAHAHAHA.