linux-kernel - Re: [PATCH 6/17] arch/x86/kernel: Add missing spin

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Date:	Tue, 1 Jun 2010 14:15:29 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Julia Lawall <julia@...u.dk>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 6/17] arch/x86/kernel: Add missing spin_unlock

On Wed, 26 May 2010 17:55:59 +0200 (CEST)
Julia Lawall <julia@...u.dk> wrote:

> From: Julia Lawall <julia@...u.dk>
> 
> Add a spin_unlock missing on the error path.  The locks and unlocks are
> balanced in other functions, so it seems that the same should be the case
> here.
> 
> The semantic match that finds this problem is as follows:
> (http://coccinelle.lip6.fr/)
> 
> // <smpl>
> @@
> expression E1;
> @@
> 
> * spin_lock(E1,...);
>   <+... when != E1
>   if (...) {
>     ... when != E1
> *   return ...;
>   }
>   ...+>
> * spin_unlock(E1,...);
> // </smpl>
> 
> Signed-off-by: Julia Lawall <julia@...u.dk>
> 
> ---
>  arch/x86/kernel/amd_iommu.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/amd_iommu.c b/arch/x86/kernel/amd_iommu.c
> index fa5a147..b98e1cd 100644
> --- a/arch/x86/kernel/amd_iommu.c
> +++ b/arch/x86/kernel/amd_iommu.c
> @@ -1499,12 +1499,16 @@ static int __attach_device(struct device *dev,
>  
>  	/* Some sanity checks */
>  	if (alias_data->domain != NULL &&
> -	    alias_data->domain != domain)
> +	    alias_data->domain != domain) {
> +		spin_unlock(&domain->lock);
>  		return -EBUSY;
> +	}
>  
>  	if (dev_data->domain != NULL &&
> -	    dev_data->domain != domain)
> +	    dev_data->domain != domain) {
> +		spin_unlock(&domain->lock);
>  		return -EBUSY;
> +	}
>  
>  	/* Do real assignment */
>  	if (dev_data->alias != dev) {

The reason why these bugs occur is that we sprinkle multiple `return'
statements inside the middle of non-trivial functions.  People miss
some or fail to modify some when later changing locking rules and we
gain bugs (or, similarly, resource leaks).

So I'd suggest that when fixing such bugs, we also fix their cause.

ie:

--- a/arch/x86/kernel/amd_iommu.c~arch-x86-kernel-add-missing-spin_unlock
+++ a/arch/x86/kernel/amd_iommu.c
@@ -1487,6 +1487,7 @@ static int __attach_device(struct device
 			   struct protection_domain *domain)
 {
 	struct iommu_dev_data *dev_data, *alias_data;
+	int ret;
 
 	dev_data   = get_dev_data(dev);
 	alias_data = get_dev_data(dev_data->alias);
@@ -1497,14 +1498,17 @@ static int __attach_device(struct device
 	/* lock domain */
 	spin_lock(&domain->lock);
 
+	ret = -EBUSY;
 	/* Some sanity checks */
 	if (alias_data->domain != NULL &&
 	    alias_data->domain != domain)
-		return -EBUSY;
+		goto out;
 
 	if (dev_data->domain != NULL &&
 	    dev_data->domain != domain)
-		return -EBUSY;
+		goto out;
+
+	ret = 0;
 
 	/* Do real assignment */
 	if (dev_data->alias != dev) {
@@ -1522,8 +1526,8 @@ static int __attach_device(struct device
 
 	/* ready */
 	spin_unlock(&domain->lock);
-
-	return 0;
+out:
+	return ret;
 }
 
 /*
_

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