Using __kuser_cmpxchg

[Nicolas Pitre]

On Fri, 16 Nov 2007, Paul Brook wrote:

> > >  > In the mean time you can work around it by using a mutex around the
> > >  > thread state modification instead of using cmpxchg on it.  Or if your
> > >  > platform is ARMv6 or higher then you won't get those spurious false
> > >  > negatives.
> > >
> > > Ah, OK, that's interesting: it sounds like you understand under what
> > > circumstances it may fail "spuriously".
> >
> > Heh, I wrote that code (and comment).  :-)
> >
> > > What are these?
> >
> > On a pre-ARMv6 processor
> 
> Similar "spurious" failures can happen on ARMv6 processors.
> 
> ARMv6 cpus don't have atomic operations either. They have ldrex/strex, where 
> the latter will fail if another CPU modifies the memory *or* a context switch 
> occurs.

Due to the current implementation this is indeed true.

ARMv6 is more trivial to fix though, since the operation can be 
reattempted whenever the strex fails.


Nicolas