Re: [PATCH] fix race in mark_mounts_for_expiry()

[David Howells <dhowells@redhat.com>]

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