[PATCH 1/4] ARM: Change the mandatory barriers implementation

[Russell King - ARM Linux]

On Tue, Feb 23, 2010 at 04:02:35PM +0000, Catalin Marinas wrote:
> > I'm not entirely convinced by the part of your patch which changes the
> > SMP barriers yet.  For instance, some drivers contain:
> > 
> >                 /* We need for force the visibility of tp->intr_mask
> >                  * for other CPUs, as we can loose an MSI interrupt
> >                  * and potentially wait for a retransmit timeout if we don't.
> >                  * The posted write to IntrMask is safe, as it will
> >                  * eventually make it to the chip and we won't loose anything
> >                  * until it does.
> >                  */
> >                 tp->intr_mask = 0xffff;
> >                 smp_wmb();
> >                 RTL_W16(IntrMask, tp->intr_event);
> > 
> > The second write is a write to hardware, and thus would be to a device
> > region.  The first is a write to a memory structure.
> > 
> > It seems to me given your description in the patch, that having smp_wmb()
> > be a dmb(), rather than a wmb() would be insufficient here.
> 
> My proposal for this would be to place an explicit DSB at the beginning
> of gic_raise_irq(). Otherwise, we can change smp_wmb() to be a DSB but
> we may have some performance penalty for other cases where ordering with
> Device accesses is not required.

That doesn't solve the above case; this isn't GIC code, it's driver code.

Given what you've said, it would appear that smp_wmb() needs to be a
wmb() in the SMP case, to ensure that the write to intr_mask is
visible to other CPUs before the interrupt mask write hits the
peripheral.

So, that leads us back to the:

#ifndef CONFIG_SMP
#define smp_mb()        barrier()
#define smp_rmb()       barrier()
#define smp_wmb()       barrier()
#else
#define smp_mb()        mb()
#define smp_rmb()       rmb()
#define smp_wmb()       wmb()
#endif

which we have at the moment.