linux-kernel - Re: Strange hang on ia64 with CONFIG_PRINTK

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Message-Id: <20080213.170452.68851396.davem@davemloft.net>
Date:	Wed, 13 Feb 2008 17:04:52 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	tony.luck@...el.com
Cc:	mingo@...e.hu, rdreier@...co.com, linux-kernel@...r.kernel.org
Subject: Re: Strange hang on ia64 with CONFIG_PRINTK_TIME=y

From: "Tony Luck" <tony.luck@...el.com>
Date: Wed, 13 Feb 2008 16:59:30 -0800

> > It is legal to access per-cpu data as early as you like,
> > it just evaluates to the static copy in the per-cpu section
> > of the kernel image until the per-cpu areas are setup.
> 
> On ia64 per-cpu variables are mapped into the top 64K
> of the address space.  Accessing them before the
> resources to handle the access to that virtual address
> have been set up would cause problems.

That's right.  I thought you guys had something that would handle that
early on, but looking at how the trick works in the vmlinux.lds.S ia64
uses that isn't the case.

I guess we really can't do per-cpu accesses early on.

This is extremely fragile, we should find some way to formalize this
to make accidental early per-cpu references trap at build time or
similar.
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