linux-kernel - Re: [git pull] scheduler fixes

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Message-ID: <alpine.LFD.1.10.0805131019360.3019@woody.linux-foundation.org>
Date:	Tue, 13 May 2008 10:22:17 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Matthew Wilcox <matthew@....cx>,
	Sven Wegener <sven.wegener@...aler.net>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [git pull] scheduler fixes

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
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