Re: [PATCH 16/17] prmem: pratomic-long

[Igor Stoppa]

On 25/10/2018 01:13, Peter Zijlstra wrote:
> On Wed, Oct 24, 2018 at 12:35:03AM +0300, Igor Stoppa wrote:
> > +static __always_inline
> > +bool __pratomic_long_op(bool inc, struct pratomic_long_t *l)
> > +{
> > +	struct page *page;
> > +	uintptr_t base;
> > +	uintptr_t offset;
> > +	unsigned long flags;
> > +	size_t size = sizeof(*l);
> > +	bool is_virt = __is_wr_after_init(l, size);
> > +
> > +	if (WARN(!(is_virt || likely(__is_wr_pool(l, size))),
> > +		 WR_ERR_RANGE_MSG))
> > +		return false;
> > +	local_irq_save(flags);
> > +	if (is_virt)
> > +		page = virt_to_page(l);
> > +	else
> > +		vmalloc_to_page(l);
> > +	offset = (~PAGE_MASK) & (uintptr_t)l;
> > +	base = (uintptr_t)vmap(&page, 1, VM_MAP, PAGE_KERNEL);
> > +	if (WARN(!base, WR_ERR_PAGE_MSG)) {
> > +		local_irq_restore(flags);
> > +		return false;
> > +	}
> > +	if (inc)
> > +		atomic_long_inc((atomic_long_t *)(base + offset));
> > +	else
> > +		atomic_long_dec((atomic_long_t *)(base + offset));
> > +	vunmap((void *)base);
> > +	local_irq_restore(flags);
> > +	return true;
> > +
> > +}
>
> That's just hideously nasty.. and horribly broken.
>
> We're not going to duplicate all these kernel interfaces wrapped in gunk
> like that. 

one possibility would be to have macros which use typeof() on the 
parameter being passed, to decide what implementation to use: regular or 
write-rare

This means that type punning would still be needed, to select the 
implementation.

Would this be enough? Is there some better way?

> Also, you _cannot_ call vunmap() with IRQs disabled. Clearly
> you've never tested this with debug bits enabled.

I thought I had them. And I _did_ have them enabled, at some point.
But I must have messed up with the configuration and I failed to notice 
this.

I can think of a way it might work, albeit it's not going to be very pretty:

* for the vmap(): if I understand correctly, it might sleep while 
obtaining memory for creating the mapping. This part could be executed 
before disabling interrupts. The rest of the function, instead, would be 
executed after interrupts are disabled.

* for vunmap(): after the writing is done, change also the alternate 
mapping to read only, then enable interrupts and destroy the alternate 
mapping. Making also the secondary mapping read only makes it equally 
secure as the primary, which means that it can be visible also with 
interrupts enabled.


--
igor