linux-kernel - Re: [tip:perfcounters/core] x86: Add NMI types for kmap

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Message-Id: <1245224675.13761.21598.camel@twins>
Date:	Wed, 17 Jun 2009 09:44:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	paulus@...ba.org, acme@...hat.com, efault@....de, npiggin@...e.de,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [tip:perfcounters/core] x86: Add NMI types for kmap_atomic

On Tue, 2009-06-16 at 10:13 +0200, Ingo Molnar wrote:

> > By removing the types it becomes very difficult to verify the max 
> > depth. I really don't like removing them.
> 
> The fact that it implies an atomic section pretty much limits its 
> depth in practice, doesnt it?
> 
> All we need to track in the debug code is 
> max-{syscall,softirq,hardirq,nmi}. The sum of these 4 counts must be 
> smaller than the max - even if (as you are right to point out) we 
> dont hit that magic combo that truly maximizes the depth.

Right, so the thing I'd worry about is someone adding kmap_atomic() to
an interrupt context that didn't have interrupts disabled and then
managing to nest that a few times.

Suppose you put it in some IO completion handler, and someone has 4 IO
controllers installed and all 4 IO interrupts come in at the 'same'
time.

With types you could warn on similarly to what we do today, but with the
simple push/pop that might be a lot harder.

Anyway, with the whole cr2 fiddling bit being discussed this seems to
become redundant.

> And note that in practice many of the current types are exclusive to 
> each other - so using the stack would _reduce_ the amount of 
> kmap-atomic space we need.

Yeah, I did realize that.
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