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

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Date:	Wed, 10 Sep 2008 08:03:18 -0700
From:	"Paul Menage" <menage@...gle.com>
To:	"Greg KH" <greg@...ah.com>
Cc:	"Li Zefan" <lizf@...fujitsu.com>,
	"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 9, 2008 at 11:29 PM, Greg KH <greg@...ah.com> wrote:
>
> But you can't put such logic in the release() function as you are
> finding out, that's not going to work either.

Right - that's why a kref_put_and_write_lock() function would be handy
- it would take the lock only when necessary. It would delegate most
of its work to the (as-yet nonexistant) atomic_dec_and_write_lock()
function, which would be an rwlock equivalent of atomic_dec_and_lock()

>
> Maybe you need to just "open-code" an atomic counter here and not use
> kref as it sounds like you are needing to do something very "special"
> here.
>

It's basically the same situation as the dentry cache - ref-counted
objects that are also referenced from a hash table protected by a
global lock.

If you're opposed to the addition of kref_put_and_write_lock() then
yes, I'll replace kref with a custom refcount.

Paul
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