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

[Russell King - ARM Linux]

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.