linux-kernel - Re: [PATCH] radix-tree: fix comment about "exceptional" bits

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Date:	Wed, 20 Jul 2016 15:10:38 +0200
From:	Johannes Thumshirn <jthumshirn@...e.de>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-nvdimm@...ts.01.org, Dave Chinner <david@...morbit.com>,
	Jan Kara <jack@...e.com>,
	Konstantin Khlebnikov <koct9i@...il.com>
Subject: Re: [PATCH] radix-tree: fix comment about "exceptional" bits

On Wed, Jul 20, 2016 at 12:55:31AM -0600, Ross Zwisler wrote:
> The bottom two bits of radix tree entries are reserved for special use by
> the radix tree code itself.  A comment detailing their usage was added by:
> 
> commit 3bcadd6fa6c4 ("radix-tree: free up the bottom bit of exceptional
> entries for reuse")
> 
> This comment states that if the bottom two bits are '11', this means that
> this is a locked exceptional entry.
> 
> It turns out that this bit combination was never actually used.  Radix tree
> locking for DAX was indeed implemented, but it actually used the third LSB:
> 
>   /* We use lowest available exceptional entry bit for locking */
>   #define RADIX_DAX_ENTRY_LOCK (1 << RADIX_TREE_EXCEPTIONAL_SHIFT)
> 
> This locking code was also made specific to the DAX code instead of being
> generally implemented in radix-tree.h.
> 
> So, fix the comment.
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>

Reviewed-by: Johannes Thumshirn <jthumshirn@...e.de>

-- 
Johannes Thumshirn                                          Storage
jthumshirn@...e.de                                +49 911 74053 689
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