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Date: Tue, 13 May 2008 10:22:17 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ingo Molnar <mingo@elte.hu>
cc: Matthew Wilcox <matthew@wil.cx>, Sven Wegener <sven.wegener@stealer.net>,
       "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
       Andi Kleen <andi@firstfloor.org>, LKML <linux-kernel@vger.kernel.org>,
       Alexander Viro <viro@ftp.linux.org.uk>,
       Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [git pull] scheduler fixes
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On Tue, 13 May 2008, Ingo Molnar wrote:
> 
> hm, this sounds like damage that is inflicted on itself by the XFS code. 

No. You're confusing what a counting semaphore is.

> Why does it signal to its waiters that "resource is available", when in 
> reality that resource is not available but immediately serialized via a 
> lock? (even if the lock might technically be some _other_ object)

So you have 'n' resources available, and you use a counting semaphore for 
that resource counting.

But you'd still need a spinlock to actually protect the list of resources 
themselves. The spinlock protects a different thing than the semaphore. 
The semaphore isn't about mutual exclusion - it's about counting resources 
and waiting when there are too many things in flight.

And you seem to think that a counting semaphore is about mutual exclusion. 
It has nothing what-so-ever to do with that. 

			Linus