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

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Date:	Tue, 9 Sep 2008 17:28:03 -0700
From:	"Paul Menage" <menage@...gle.com>
To:	"Lai Jiangshan" <laijs@...fujitsu.com>
Cc:	"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 Mon, Aug 18, 2008 at 11:29 PM, Lai Jiangshan <laijs@...fujitsu.com> wrote:
> put_css_set_taskexit may be called when find_css_set is called on
> other cpu. And the race will occur:
>


Sorry I didn't respond to this when it originally came out - I was on vacation.

I agree that it's a race that needs to be fixed, but I'm not sure that
I like the fix that can generate kref warnings.

I can see two possible fixes:

1) avoid the race entirely by introducing some new primitives:

atomic_dec_and_write_lock() (like atomic_dec_and_lock(), but for an rwlock)

and kref_put_and_write_lock() which would be something like:

int kref_put_and_write_lock(struct kref *kref, void (*release)(struct
kref *kref), rwlock*lock)
{
  if(atomic_dec_and_write_lock(&kref->refcount, lock)) {
    release(kref);
    return 1;
  }
  return 0;
}

We'd then use kref_put_and_write_lock(), and enter __release_css_set()
with the lock already held

2) Use atomic_inc_not_zero() in find_existing_css_set(), to ensure
that we only return a referenced css, and remove the get_css_set()
call from find_css_set(). (Possibly wrapping this in a new
kref_get_not_zero() function)

Paul
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