From mboxrd@z Thu Jan  1 00:00:00 1970
From: Roland Dreier <rdreier@cisco.com>
Subject: Re: [PATCH] fs: O_* bit numbers uniqueness check
Date: Tue, 05 Jan 2010 23:08:08 -0800
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Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Miller <davem@davemloft.net>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Christoph Hellwig <hch@infradead.org>,
	Eric Paris <eparis@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org
To: Wu Fengguang <fengguang.wu@intel.com>
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 > +	/* please add new bits here to ensure allocation uniqueness */
 > +	BUG_ON(20 != hweight32(
 > +		O_RDONLY	| O_WRONLY	| O_RDWR	|

I wonder if there's a way to make this BUILD_BUG_ON(), so the problem is
caught at compile time (a change like adding an O_ flag is often compile
tested only for obscure archs).  One could create a compile-time
macro-ized version of hweight32() and use that, I guess, although I'm
not sure it's worth the ugliness.

Failing that maybe this should be WARN_ON()?  I'd be annoyed if my arch
wouldn't boot because some strange new O_ flag happened to collide.

 - R.