RE: Synchronizing Bit operations V2

[Chen, Kenneth W]

Christoph Lameter wrote on Thursday, March 30, 2006 6:56 PM
> > By the way, this is the same thing on x86: look at include/asm-i386/bitops.h:
> > 
> > #define smp_mb__before_clear_bit()      barrier()
> > #define smp_mb__after_clear_bit()       barrier()
> > 
> > A simple compiler barrier, nothing but
> > #define barrier() __asm__ __volatile__("": : :"memory")
> > 
> > See, no memory ordering there, because clear_bit already has a LOCK prefix.
> 
> And that implies barrier behavior right?

No, not the memory ordering semantics you are thinking about.  It just tell
compiler not to be over smart and schedule a load operation above that point
Intel compiler is good at schedule memory load way ahead of its use to hide
memory latency. gcc probably does that too, I'm not 100% sure. This prevents
the compiler to schedule load before that line.

- Ken
-
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/