Re: [PATCH 10/20] powerpc/dma-noncoherent: don't disable irqs over kmap_atomic

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2018-08-22 at 09:02 +0200, Christoph Hellwig wrote:
> 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.

This is the same with x86 no ?

32-bit x86 kmap_atomic_prot is the same as ours...

In fact I wonder why the preempt_disable() in there since it needs to
be protected against interrupt ?

Or is it that we never actually call kmap_atomic_* these days from
interrupt, and the atomic versions are just about dealing with
spinlocks ?

Cheers,
Ben.