From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
Subject: Re: [PATCH 10/20] powerpc/dma-noncoherent: don't disable irqs over
	kmap_atomic
Date: Wed, 22 Aug 2018 09:02:41 +0200
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To: Benjamin Herrenschmidt <benh-XVmvHMARGAS8U2dJNN8I7kB+6BGkLq7r@public.gmane.org>
Cc: Fenghua Yu <fenghua.yu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>, Tony Luck <tony.luck-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>, linux-ia64-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Konrad Rzeszutek Wilk <konrad.wilk-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>, Michael Ellerman <mpe-Gsx/Oe8HsFggBc27wqDAHg@public.gmane.org>, linuxppc-dev-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Paul Mackerras <paulus-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>, Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>, Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
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On Thu, Aug 09, 2018 at 10:27:46AM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2018-07-30 at 18:38 +0200, Christoph Hellwig wrote:
> > The requirement to disable local irqs over kmap_atomic is long gone,
> > so remove those calls.
> 
> Really ? I'm trying to verify that and getting lost in a mess of macros
> from hell in the per-cpu stuff but if you look at our implementation
> of kmap_atomic_prot(), all it does is a preempt_disable(), and then
> it uses kmap_atomic_idx_push():
> 
> 	int idx = __this_cpu_inc_return(__kmap_atomic_idx) - 1;
> 
> Note the use of __this_cpu_inc_return(), not this_cpu_inc_return(),
> ie this is the non-interrupt safe version...

Looks like the powerpc variant indeed isn't save.

I did look a bit more through the code and history, and it seems
like we remove the need to disable irqs when called from process
context a while ago, but we still require disabling irqs when called
from irq context.  Given that this code can also be called from
irq context we'll have to keep the local_irq_save.