linux-ext4 - Re: [PATCH 5/6] ext4: Drop i_state_flags on architectures with 64-bit longs

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Message-ID: <20110105202952.GO2959@thunk.org>
Date:	Wed, 5 Jan 2011 15:29:52 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 5/6] ext4: Drop i_state_flags on architectures with
 64-bit longs

On Wed, Jan 05, 2011 at 11:43:08AM -0700, Andreas Dilger wrote:
> 
> It looks like you have compensated for the above by changing the
> code here, but I think it is risky/confusing if clearing the state
> flags has a side-effect on 64-bit arches, that doesn't exist on
> 32-bit arches.  It looks like a bug waiting to happen...

Yeah, I did think of this, but it seemed like extra/needless work that
I was trying to optimize away.  It's still not safe to do:

#define EXT4_CLEAR_STATE_FLAGS(ei)      (ei)->i_flags &= 0xffffffffULL;

... since we're not atomically updating i_flags.  So if anyone tries
using EXT4_CLEAR_STATE_FLAGS() aside from the two allocation contexts,
they need to be careful anyway.

I did think about putting the #ifdef BITS_PER_LONG < 64 inline in the
code, but that's ugly.  

Maybe the best thing to do is to clearly document this pitfall, and
then leave things as-is?   There aren't a lot of great solutions.

						- Ted
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