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

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Date:	Wed, 22 Jul 2015 13:38:48 +0800
From:	Pan Xinhui <xinhuix.pan@...el.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	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: [PATCH V3] x86/mm/pat: Do a small optimization and fix in reserve_memtype

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(-)

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;
+	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
--
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