mm: get_user_pages_fast()

[Russell King - ARM Linux]

On Tue, Nov 05, 2013 at 10:43:15PM +0000, Michael McTernan wrote:
> I think there's a problem on ARM with futex calls to FUTEX_WAIT and  
> similar for addresses that have been mapped to userspace from /dev/mem  
> or a device which sets VM_IO or VM_PFNMAP.  This can break  
> pthread_mutex_lock() if placed on such mapped memory and attributed as  
> process-shared.

Firstly, please stop this madness.  Placing futexes in IO memory is not
supported, period.  You can't map random bits of /dev/mem and expect
this stuff to work.

(a) you're generally not allowed to map kernel memory via /dev/mem, so
that rules out all kernel managed memory.
(b) you are allowed to map memory outside of that, but you get it as
strongly ordered memory.

(b) means that the load/store exclusives, which userspace mutexes will
use on ARMv6+, will not work correctly.  (they're not supported to
strongly ordered memory by the architecture.)

In other words, don't put mutexes in memory you've remapped from /dev/mem.
In fact, as an application, you should not be mapping /dev/mem at all.