[PATCH] ARM: Add PI/robust mutexes support for SMP kernels

[Catalin Marinas]

On Mon, 2010-06-07 at 20:44 +0100, Russell King - ARM Linux wrote:
> On Mon, Jun 07, 2010 at 09:36:30PM +0400, Anton Vorontsov wrote:
> > To support PI or robust mutexes, the kernel needs to perform some
> > operations atomically on userspace addresses, and yet ARM lacked
> > the support for the SMP case.
> >
> > ARMv6 adds exclusive access variants of ldr and str instructions,
> > which means that support for PI/robust mutexes should now be
> > relatively straightforward.
> 
> It isn't this straight forward.  You're now bypassing the MMU protections
> in that 'strex' can bypass the read-only protection of the user page.
> This can result in the zero BSS page being corrupted, or worse corruption
> to page cache pages.

Do you mean STREX in SVC mode targeting user pages? I keep posting a
patch for more than a year :) which removes the domains switching and
makes user pages read-only for both kernel and user space.

Making user RO / kernel RO pages doesn't necessarily require the full
domains patch, just the parts that deal with the vectors page and FIQ
setting (though we really need the whole domains patch in the kernel for
many reasons).

Such permissions are also required for Leif's patch which handles SWP
emulation via STREX/LDREX.

-- 
Catalin