linux-kernel - Re: [PATCH 1/4] eventfd: introduce eventfd_signal

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Date:	Tue, 5 Feb 2013 11:40:50 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Aaron Durbin <adurbin@...gle.com>,
	Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH 1/4] eventfd: introduce eventfd_signal_hangup()

On 2013/2/4 18:15, Kirill A. Shutemov wrote:
> On Sat, Feb 02, 2013 at 05:58:58PM +0200, Kirill A. Shutemov wrote:
>> On Sat, Feb 02, 2013 at 02:50:44PM +0800, Li Zefan wrote:
>>> When an eventfd is closed, a wakeup with POLLHUP will be issued,
>>> but cgroup wants to issue wakeup explicitly, so when a cgroup is
>>> removed userspace can be notified.
>>>
>>> Signed-off-by: Li Zefan <lizefan@...wei.com>
> 
> Hm.. Looks like it will break eventfd semantics:
> 
> 1. One eventfd can be used for deliver more then one notification from
> one or more cgroups. POLLHUP on removing one of cgroups is not valid.
> 
> 2. It's valid to have eventfd opened only by one userspace application. We
> should not close it, just because cgroup is removed.
> 
> I think problem with multiple threads waiting an event on eventfd should
> be handled in userspace.
> 

I didn't realize this.. and if a cgroup is removed, the woken thread may not
be the thread that is waiting on this cgroup. How crappy.. I don't know how
userspace is going to deal with all these.

And another bug spotted. We can pass fd of memory.usage_in_bytes of cgroup A
to cgroup.event_control of cgroup B, and then we won't get memory usage
notification from A but B! What's worse, if A and B are in different mount
hierarchy, boom!


Signed-off-by: Li Zefan <lizefan@...wei.com>
---
 kernel/cgroup.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3d21adf..e496359 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3825,6 +3825,7 @@ static int cgroup_write_event_control(struct cgroup *cgrp, struct cftype *cft,
 				      const char *buffer)
 {
 	struct cgroup_event *event = NULL;
+	struct cgroup *cgrp_cfile;
 	unsigned int efd, cfd;
 	struct file *efile = NULL;
 	struct file *cfile = NULL;
@@ -3880,6 +3881,16 @@ static int cgroup_write_event_control(struct cgroup *cgrp, struct cftype *cft,
 		goto fail;
 	}
 
+	/*
+	 * The file to be monitored must be in the same cgroup as
+	 * cgroup.event_control is.
+	 */
+	cgrp_cfile = __d_cgrp(cfile->f_dentry->d_parent);
+	if (cgrp_cfile != cgrp) {
+		ret = -EINVAL;
+		goto fail;
+	}
+
 	if (!event->cft->register_event || !event->cft->unregister_event) {
 		ret = -EINVAL;
 		goto fail;
-- 
1.8.0.2


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