linux-kernel - Re: [PATCH v5 1/4] cgroup: implement eventfd-based generic API for notifications

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Date:	Fri, 08 Jan 2010 08:55:40 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	Paul Menage <menage@...gle.com>,
	containers@...ts.linux-foundation.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Dan Malek <dan@...eddedalley.com>,
	Vladislav Buzov <vbuzov@...eddedalley.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Alexander Shishkin <virtuoso@...nd.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/4] cgroup: implement eventfd-based generic API for
 	notifications

Kirill A. Shutemov wrote:
> On Thu, Jan 7, 2010 at 3:01 AM, Paul Menage <menage@...gle.com> wrote:
>> On Wed, Dec 30, 2009 at 7:57 AM, Kirill A. Shutemov
>> <kirill@...temov.name> wrote:
>>> +
>>> +       if (!IS_ERR(efile))
>>> +               fput(efile);
>> While this is OK currently, it's a bit fragile. efile starts as NULL,
>> and IS_ERR(NULL) is false. So if we jump to fail: before trying to do
>> the eventfd_fget() then we'll try to fput(NULL), which will oops. This
>> works because we don't currently jump to fail: until after
>> eventfd_fget(), but someone could add an extra setup step between the
>> kzalloc() and the eventfd_fget() which could fail.
> 
> So we need to use IS_ERR_OR_NULL here instread of IS_ERR, don't we?
> 

Use multi labels is much better:

label4::
	fput(cfile);
label3:
	eventfd_ctx_put(event->eventfd);
label2:
	fput(efile);
label1:
	kfree(event);

compared to:

+fail:
+	if (!IS_ERR(cfile))
+		fput(cfile);
+
+	if (event && event->eventfd && !IS_ERR(event->eventfd))
+		eventfd_ctx_put(event->eventfd);
+
+	if (!IS_ERR(efile))
+		fput(efile);
+
+	kfree(event);

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