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

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Date:	Thu, 10 Jul 2008 16:59:34 -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, 10 Jul 2008 18:46:44 +0200 Joerg Roedel <joerg.roedel@....com> wrote:

> > > +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?
> 
> This function is called from the dma-mapping path. As far as I know the
> DMA mapping functions can be called from interrupt 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.
> 
> The lock only protects the protection domain table (and the device
> table) itself. It does not protect the values the pointers in that list
> point to. In this case its also not racy because a value to that list is
> only written once and then never changed again (currently). If this
> changes in the future (and it will) I will change the locking too. This
> will also need reference counting for 'struct protection_domain' which
> is not implemented yet.

So no locking is needed in this function.
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