linux-kernel - Re: [RFC][PATCH 2/2] ARM: ioremap: Add IO mapping space reused support.

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Date:	Thu, 15 May 2014 11:17:34 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Richard Lee <superlibj8301@...il.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org, linux@....linux.org.uk,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Richard Lee <superlibj@...il.com>
Subject: Re: [RFC][PATCH 2/2] ARM: ioremap: Add IO mapping space reused
 support.

On Tue, May 13, 2014 at 09:45:08AM +0800, Richard Lee wrote:
> > On Mon, May 12, 2014 at 3:51 PM, Arnd Bergmann <arnd@...db.de> wrote:
> > On Monday 12 May 2014 10:19:55 Richard Lee wrote:
> >> For the IO mapping, for the same physical address space maybe
> >> mapped more than one time, for example, in some SoCs:
> >> 0x20000000 ~ 0x20001000: are global control IO physical map,
> >> and this range space will be used by many drivers.
> >> And then if each driver will do the same ioremap operation, we
> >> will waste to much malloc virtual spaces.
> >>
> >> This patch add IO mapping space reused support.
> >>
> >> Signed-off-by: Richard Lee <superlibj@...il.com>
> >
> > What happens if the first driver then unmaps the area?
> 
> If the first driver will unmap the area, it shouldn't do any thing
> except decreasing the 'used' counter.

It's still racy. What if the first driver manage to decrement the used
counter, unmaps the regions but doesn't yet free the vm_struct while
another driver finds the vm_struct, increments the used count and
assumes it can use it?

BTW, vm_area_is_aready_to_free() name implies a query but it has
side-effects like decrementing the counter.

-- 
Catalin
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