linux-kernel - Re: [RFC][PATCH] kmap_atomic

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Date:	Mon, 12 Oct 2009 20:40:29 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	David Howells <dhowells@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC][PATCH] kmap_atomic_push

On Mon, Oct 12, 2009 at 11:30:07AM -0700, Linus Torvalds wrote:
> 
> 
> On Mon, 12 Oct 2009, Andi Kleen wrote:
> 
> > Peter Zijlstra <peterz@...radead.org> writes:
> > > -
> > > -static inline void debug_kmap_atomic(enum km_type type)
> > > +static inline int kmap_atomic_push_idx(void)
> > >  {
> > > +	int idx = __get_cpu_var(__kmap_atomic_depth)++;
> > 
> > The counter needs to be of local atomic type. Otherwise kmap_atomic cannot
> > be done from interrupts/nmis, which is unfortunately occasionally needed.
> 
> I thought so too on lookin gat it initially, but it's not actually true.
> 
> It's both IRQ and NMI safe as-is, for a very simple reason: any interrupts 

Good point, thanks.

I was thinking of CPU migration in interrupt cases, but even there
it should be ok in mainline.

I suppose it's not true for the preempt-rt folks (who can migrate
CPUs at any time), so it might be still more friendly to handle it for them 
though.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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