linux-kernel - Re: memory barrier to ensure copy_to

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Date:	Fri, 31 Aug 2007 17:08:07 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	"anon... anon.al" <anon.asdf@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: memory barrier to ensure copy_to_user() completes

> Which memory barrier do I require if I need to ensure that a
> copy_to_user(dest, src, len) completes before the next statement?

Define "completes"

> 
> copy_to_user(dest, src, len) ;
> //rmb(); OR wmb(); OR barrier(); OR mb(); ??????

Usually none of them

> If I'm writing to hardware, and need to ensure the correct order, I'll
> use wmb(), right?

No

> e.g.:
> 
> #define HW_address1 20
> #define HW_address2 40
> 
> *((int *)HW_address1) = 0x00000001;
> wmb();  // is this good???
> *((int *)HW_address2) = 0x00000010;

Linux doesn't define directly poking kernel addresses to hit hardware as
valid (and on many platforms it doesn't work). Use readl/writel and their
ordering is defined.

Read Documentation/io_ordering.txt and Docuemntation/pci.txt
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