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Date: Tue, 27 May 2008 10:06:11 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@tv-sign.ru>, Alexey Dobriyan <adobriyan@gmail.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: 2.6.26-rc4: RIP find_pid_ns+0x6b/0xa0
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References: <alpine.LFD.1.10.0805261127030.2958@woody.linux-foundation.org> <20080527052318.GA28301@martell.zuzino.mipt.ru> <20080527090605.GA146@tv-sign.ru> <alpine.LFD.1.10.0805270746090.2958@woody.linux-foundation.org> <20080527154001.GB14296@linux.vnet.ibm.com> <alpine.LFD.1.10.0805270908250.2958@woody.linux-foundation.org>
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On Tue, May 27, 2008 at 09:11:33AM -0700, Linus Torvalds wrote:
> On Tue, 27 May 2008, Paul E. McKenney wrote:
> > 
> > But this will only help until preemptible spinlocks arrive, right?
> 
> I don't think we will ever have preemptible spinlocks.
> 
> If you preempt spinlocks, you have serious issues with contention and 
> priority inversion etc, and you basically need to turn them into sleeping 
> mutexes. So now you also need to do interrupts as sleepable threads etc 
> etc.

Indeed, all of these are required in that case.

> And it would break the existing non-preempt RCU usage anyway.

Yes, preemptable spinlocks cannot work without preemptable RCU.

> Yeah, maybe the RT people try to do that, but quite frankly, it is insane. 
> Spinlocks are *different* from sleeping locks, for a damn good reason.

Well, I guess I never claimed to be sane...

Anyway, will look at a preemptable RCU that waits for preempt-disable
sections of code.

						Thanx, Paul