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Date: Mon, 30 Nov 2009 18:46:38 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@linux-foundation.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Thomas Gleixner <tglx@linutronix.de>
Cc: Christoph Hellwig <hch@infradead.org>, Nick Piggin <npiggin@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [rfc] "fair" rw spinlocks
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, 30 Nov 2009, Christoph Hellwig wrote:
> > 
> > How long will this use be around?  I've seen some slow progress toward
> > replacing most read side uses of the task list lock with RCU.  While we
> > still have lots of read side users now I wonder when they'll go away.
> 
> tasklist_lock is pretty nasty. I threw out "replace it with RCU" 
> because it would be nice, but the data structures used are not just 
> simple linked lists that we have RCU helpers for traversing.
> 
> There are various real exclusion rules about things like 
> 'tsk->exit_state' etc, which do not translate directly to RCU usage. 
> Of course, _maybe_ all the places that care already take the thing for 
> writing and would just automatically have exclusion anyway.
> 
> So I'd love to see somebody try to do the conversion. To a first 
> approximation, you probably could do
> 
>  - turn tasklist_lock into a spinlock
> 
>  - sed 's/write_lock_irq(&tasklist_lock)/spin_lock(&tasklist_lock)/g'
>    sed 's/write_unlock_irq(&tasklist_lock)/spin_unlock(&tasklist_lock)/g'
> 
>  - sed 's/read_lock(&tasklist_lock)/rcu_read_lock()/g'
>    sed 's/read_unlock(&tasklist_lock)/rcu_read_unlock()/g'
> 
>  - make all the task lists use the RCU versions of the list routines
> 
>  - free the task structure using RCU
> 
> and you'd be _pretty_ close to a working system.

In -rt we've got that in essence, and it's indeed working fine (with a 
few caveats). A few RCU conversions of tasklist_lock usage in that area 
even trickled upstream, because the simple lock would hurt so much under 
-rt.

	Ingo