linux-kernel - Re: [PATCH] memcg: Only free spare array when readers are done

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Message-ID: <20160115205653.GA26524@cmpxchg.org>
Date:	Fri, 15 Jan 2016 15:56:53 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Martijn Coenen <maco@...gle.com>
Cc:	Michal Hocko <mhocko@...nel.org>,
	Vladimir Davydov <vdavydov@...tuozzo.com>,
	cgroups@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcg: Only free spare array when readers are done

On Thu, Jan 14, 2016 at 02:33:52PM +0100, Martijn Coenen wrote:
> A spare array holding mem cgroup threshold events is kept around
> to make sure we can always safely deregister an event and have an
> array to store the new set of events in.
> 
> In the scenario where we're going from 1 to 0 registered events, the
> pointer to the primary array containing 1 event is copied to the spare
> slot, and then the spare slot is freed because no events are left.
> However, it is freed before calling synchronize_rcu(), which means
> readers may still be accessing threshold->primary after it is freed.
> 
> Fixed by only freeing after synchronize_rcu().
> 
> Signed-off-by: Martijn Coenen <maco@...gle.com>

Acked-by: Johannes Weiner <hannes@...xchg.org>