linux-kernel - Re: [PATCH] cgroups: fix probable race with put_css_set[_taskexit] and find_css

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Message-ID: <20080910061717.GA6301@kroah.com>
Date:	Tue, 9 Sep 2008 23:17:17 -0700
From:	Greg KH <greg@...ah.com>
To:	Paul Menage <menage@...gle.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cgroups: fix probable race with put_css_set[_taskexit]
	and find_css_set

On Tue, Sep 09, 2008 at 10:31:24PM -0700, Paul Menage wrote:
> On Tue, Sep 9, 2008 at 10:01 PM, Greg KH <greg@...ah.com> wrote:
> >
> > What are you trying to solve here with this change?  I agree, it does
> > seem a bit "chaotic" :)
> 
> There's a place in cgroups that uses kref_put() to release an object;
> the release function *then* takes a write-lock and removes the object
> from a lookup table; it could race with another thread that searches
> the lookup table (while holding a read-lock) and does kref_get() on
> the same object.

Ick, yeah that's not good.

What about the way everyone else solves this, grab the lock before you
call kref_put()?

> The current fix is for the release function to recheck inside the lock
> that the object's refcount is still zero, and only actually
> unlink/free it if so. And actually I've just realised that this isn't
> actually even safe, since the thread that just acquired the object
> could kref_put() it almost immediately, which would leave two threads
> both trying to unlink/free the object.

Yeah, don't do that :)

thanks,

greg k-h
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