linux-kernel - Re: [PATCH V3] x86/mm/pat: Do a small optimization and fix in reserve

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Date:	Wed, 22 Jul 2015 09:46:49 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Pan Xinhui <xinhuix.pan@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, bp@...e.de, toshi.kani@...com,
	jgross@...e.com, mcgrof@...e.com,
	"mnipxh@....com" <mnipxh@....com>,
	"yanmin_zhang@...ux.intel.com" <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH V3] x86/mm/pat: Do a small optimization and fix in
 reserve_memtype

On Wed, Jul 22, 2015 at 01:38:48PM +0800, Pan Xinhui wrote:
> From: Pan Xinhui <xinhuix.pan@...el.com>
> 
> It's more reasonable to unlock memtype_lock right after
> rbt_memtype_check_insert. memtype_lock protects all data stored in
> rb-tree from multiple access. It's not cool to call kfree, pr_info, etc
> with this lock held. So move spin_unlock a little ahead.
> 
> If *new* succeed to be stored into the rb-tree, we might hit panic.
> Because we access *new* in dprintk "cattr_name(new->type)". Data stored
> in the rb-tree might be freed at any possbile time. It's abviously wrong
> to access such data without lock held. As new->type might be changed in
> rbt_memtype_check_insert, so save new->type to actual_type, then use
> actual_type in dprintk.
> 
> Signed-off-by: Pan Xinhui <xinhuix.pan@...el.com>
> ---
> change from v2:
> 	update comments.
> change from V1:
> 	fix an access of *new* without memtype_lock held.
> ---
>  arch/x86/mm/pat.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)

This patch still doesn't update the comments over memtype_lock.

> 
> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> index 188e3e0..894a096 100644
> --- a/arch/x86/mm/pat.c
> +++ b/arch/x86/mm/pat.c
> @@ -538,22 +538,25 @@ int reserve_memtype(u64 start, u64 end, enum page_cache_mode req_type,
>  	new->type	= actual_type;
>  
>  	spin_lock(&memtype_lock);
> -
>  	err = rbt_memtype_check_insert(new, new_type);
> +	/*
> +	 * new->type might be changed in rbt_memtype_check_insert.
> +	 * So save new->type to actual_type as dprintk uses it.
> +	 * We are not allowed to touch new after unlocking memtype_lock.
> +	 */
> +	actual_type = new->type;

We already assign actual_type to new->type above. I think the dprintk
needs actual_type and not what new->type has been changed to as that is
in new_type.

> +	spin_unlock(&memtype_lock);
> +
>  	if (err) {
>  		pr_info("x86/PAT: reserve_memtype failed [mem %#010Lx-%#010Lx], track %s, req %s\n",
>  			start, end - 1,
>  			cattr_name(new->type), cattr_name(req_type));
>  		kfree(new);
> -		spin_unlock(&memtype_lock);
> -
>  		return err;
>  	}
>  
> -	spin_unlock(&memtype_lock);
> -
>  	dprintk("reserve_memtype added [mem %#010Lx-%#010Lx], track %s, req %s, ret %s\n",
> -		start, end - 1, cattr_name(new->type), cattr_name(req_type),
> +		start, end - 1, cattr_name(actual_type), cattr_name(req_type),
>  		new_type ? cattr_name(*new_type) : "-");
>  
>  	return err;
> -- 
> 1.9.1

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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