[PATCH] Document Linux's memory barriers

[Matthew Wilcox]

On Tue, Mar 07, 2006 at 01:54:33PM -0500, linux-os (Dick Johnson) wrote:
> This might be a good place to document:
>     dummy = readl(&foodev->ctrl);
> 
> Will flush all pending writes to the PCI bus and that:
>     (void) readl(&foodev->ctrl);
> ... won't because `gcc` may optimize it away. In fact, variable
> "dummy" should be global or `gcc` may make it go away as well.

static inline unsigned int readl(const volatile void __iomem *addr)
{
	return *(volatile unsigned int __force *) addr;
}

The cast is volatile, so gcc knows not to optimise it away.