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

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Date:	Wed, 6 Jan 2010 17:01:21 -0800
From:	Paul Menage <menage@...gle.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	containers@...ts.linux-foundation.org, linux-mm@...ck.org,
	Li Zefan <lizf@...fujitsu.com>,
	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

On Wed, Dec 30, 2009 at 7:57 AM, Kirill A. Shutemov
<kirill@...temov.name> wrote:
> This patch introduces write-only file "cgroup.event_control" in every
> cgroup.

This looks like a nice generic API for doing event notifications - thanks!

Sorry I hadn't had a chance to review it before now, due to travelling
and day-job pressures.

> +}
> +
> +static int cgroup_event_wake(wait_queue_t *wait, unsigned mode,
> +               int sync, void *key)

Maybe some comments here indicating how/when it gets called? (And more
comments for each function generally?)

> +       if (flags & POLLHUP) {
> +               spin_lock(&cgrp->event_list_lock);
> +               list_del(&event->list);
> +               spin_unlock(&cgrp->event_list_lock);
> +               schedule_work(&event->remove);

Comment saying why we can't do the remove immediately in this context?

> +
> +fail:
> +       if (!IS_ERR(cfile))
> +               fput(cfile);

cfile is either valid or NULL - it never contains an error value.

> +
> +       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.

Paul
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