Re: NIU driver: Sun x8 Express Quad Gigabit Ethernet Adapter

David Miller
2008-11-12T21:46:18Z

From: Ben Hutchings <bhutchings@solarflare.com>
Date: Wed, 12 Nov 2008 12:54:53 +0000

> On Wed, 2008-11-12 at 04:11 -0800, David Miller wrote:
> [...]
> > So the following patch should fix this bug. writeq() should
> > be OK as-is, so doesn't need a similar change.
> >
> > diff --git a/drivers/net/niu.c b/drivers/net/niu.c
> > index 9acb5d7..d8463b1 100644
> > --- a/drivers/net/niu.c
> > +++ b/drivers/net/niu.c
> > @@ -51,8 +51,7 @@ MODULE_VERSION(DRV_MODULE_VERSION);
> > #ifndef readq
> > static u64 readq(void __iomem *reg)
> > {
> > - return (((u64)readl(reg + 0x4UL) << 32) |
> > - (u64)readl(reg));
> > + return ((u64) readl(reg)) | (((u64) readl(reg + 4UL)) << 32);
> > }
>
> Since there's no sequence point between the reads, there's no guarantee
> that the reads happen in the order written (regardless of barriers
> inside readl()). This needs to be split into two statements.

What version of the C language are you using?

I personally think it's safe. If the compiler sees "A | B" it's going
to emit the code to compute A, then the code to emit B, and finally
the "|" operation.

Everything I've always seen says that for "|" the expressions are
evaluated left to right.