Re: Broken ARM (and powerpc ?) futex wrt memory barriers

[Russell King - ARM Linux]

On Mon, May 25, 2009 at 05:45:33PM -0400, Mathieu Desnoyers wrote:
> I think we also have to deal with futexes. See
> arch/arm/include/asm/futex.h :
> 
> 1 - 
> 
> #ifdef CONFIG_SMP
> 
> #include <asm-generic/futex.h>
> 
> #else /* !SMP, we can work around lack of atomic ops by disabling
> preemption */
> 
> (arm-specific code here, seems to deal with futexes)
> 
> #endif
> 
> -> is it just me or this ifdef condition is the exact opposite of what
> it should be ? I thought those generic futexes were for UP-only
> systems...

No, it is correct.  Look deeper - the generic stuff implements a whole
load of futex stuff as returning -ENOSYS - it's not implemented.

What you see in ARM is the UP code implementing the set/add/or/andn/xor
cmpxchg operations.  However, this code is most certainly not SMP safe
(and I couldn't persuade the folk writing it to come up with a SMP
version.)  So, on ARM futex is supported on UP systems, but not SMP
systems until someone gets around to fixing this.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/