From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Howells <dhowells@redhat.com>
Subject: Re: [PATCH] fix race in mark_mounts_for_expiry()
Date: Wed, 18 May 2005 11:46:15 +0100
Message-ID: <7230.1116413175@redhat.com>
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In-Reply-To: <E1DYLvb-0000as-00@dorka.pomaz.szeredi.hu> 
To: Miklos Szeredi <miklos@szeredi.hu>
Sender: linux-fsdevel-owner@vger.kernel.org
List-Id: linux-fsdevel.vger.kernel.org

Miklos Szeredi <miklos@szeredi.hu> wrote:

> > How about using cmpxchg?
> 
> How?  If the count is nonzero, an incremented count must be stored.
> You can't do that atomically with cmpxchg.

Yes you can. cmpxchg() is atomic. Several archs implement atomic_inc() and co
with cmpxchg() or similar.

Something like:

	static inline struct namespace *grab_namespace(struct namespace *n)
	{
		int old = atomic_read(&n->count);

		while (old > 0) {
			/* attempt to increment the counter */
			old = cmpxchg(&n->count, old, old + 1);
		}

		return old > 0 ? n : NULL;
	}

David