Date: Wed, 17 Jun 2009 14:28:12 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>, linux-kernel@vger.kernel.org,
       mingo@redhat.com, hpa@zytor.com, paulus@samba.org, acme@redhat.com,
       efault@gmx.de, npiggin@suse.de, tglx@linutronix.de,
       linux-tip-commits@vger.kernel.org,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [tip:perfcounters/core] x86: Add NMI types for kmap_atomic
Message-ID: <20090617122812.GD13316@elte.hu>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> 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.

Yes, fixed-purpose allocations are easier to warn about - they imply 
more constraints, no doubt about that.

But we could warn about using kmap-atomic with in irq context with 
irqs enabled and thus exclude the case you are worried about?

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

It's not just the cr2 fiddling but also conversion of pagefault 
returns from IRET to RET. The kmap_atomic API change is a nice 
cleanup in itself.

	Ingo
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