linux-kernel - Re: [PATCH 23/34] AMD IOMMU: add functions to find IOMMU device resources

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Date:	Wed, 9 Jul 2008 19:18:27 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Joerg Roedel <joerg.roedel@....com>
Cc:	tglx@...utronix.de, mingo@...hat.com, linux-kernel@...r.kernel.org,
	iommu@...ts.linux-foundation.org, bhavna.sarathy@....com,
	Sebastian.Biemueller@....com, robert.richter@....com,
	joro@...tes.org
Subject: Re: [PATCH 23/34] AMD IOMMU: add functions to find IOMMU device
 resources

On Thu, 26 Jun 2008 21:27:59 +0200 Joerg Roedel <joerg.roedel@....com> wrote:

> This patch adds functions necessary to find the IOMMU resources for a specific
> device.
> 
> Signed-off-by: Joerg Roedel <joerg.roedel@....com>
> ---
>  arch/x86/kernel/amd_iommu.c |   75 +++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 75 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/amd_iommu.c b/arch/x86/kernel/amd_iommu.c
> index c43d15d..47e80b5 100644
> --- a/arch/x86/kernel/amd_iommu.c
> +++ b/arch/x86/kernel/amd_iommu.c
> @@ -461,3 +461,78 @@ free_dma_dom:
>  	return NULL;
>  }
>  
> +static struct protection_domain *domain_for_device(u16 devid)
> +{
> +	struct protection_domain *dom;
> +	unsigned long flags;
> +
> +	read_lock_irqsave(&amd_iommu_devtable_lock, flags);

Why is this cheerfully undocumented lock irq-safe?  Is it ever taken from
IRQ context?

> +	dom = amd_iommu_pd_table[devid];
> +	read_unlock_irqrestore(&amd_iommu_devtable_lock, flags);
> +
> +	return dom;
> +}

The locking in this function makes no sense.  We drop the lock then return
a value which the caller cannot use in a race-free fashion, because the
lock is no longer held.


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