linux-kernel - Re: [PATCH] ARM: SMP: prevent use of the console when using idmap

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Date:	Tue, 8 May 2012 10:21:09 +0100
From:	Will Deacon <will.deacon@....com>
To:	Colin Cross <ccross@...roid.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@....linux.org.uk>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: SMP: prevent use of the console when using idmap_pgd

Hi Colin,

Sorry for the delayed response.

On Sat, May 05, 2012 at 02:14:02AM +0100, Colin Cross wrote:
> Commit 4e8ee7de227e3ab9a72040b448ad728c5428a042 (ARM: SMP: use
> idmap_pgd for mapping MMU enable during secondary booting)
> switched secondary boot to use idmap_pgd, which is initialized
> during early_initcall, instead of a page table initialized during
> __cpu_up.  This causes idmap_pgd to contain the static mappings
> but be missing all dynamic mappings.
> 
> If a console is registered that creates a dynamic mapping, the
> printk in secondary_start_kernel will trigger a data abort on
> the missing mapping before the exception handlers have been
> initialized, leading to a hang.  Initial boot is not affected
> because no consoles have been registered, and resume is usually
> not affected because the offending console is suspended.
> Onlining a cpu with hotplug triggers the problem.
> 
> A workaround is to the printk in secondary_start_kernel until
> after the page tables have been switched back to init_mm.

You missed a word out here (`delay'?) but it looks like Russell has taken
this now so I'm sure we'll cope.

Acked-by: Will Deacon <will.deacon@....com>

Will
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