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

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Message-Id: <1468997731-2155-1-git-send-email-ross.zwisler@linux.intel.com>
Date:	Wed, 20 Jul 2016 00:55:31 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	linux-kernel@...r.kernel.org
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
	linux-nvdimm@...ts.01.org, Konstantin Khlebnikov <koct9i@...il.com>
Subject: [PATCH] radix-tree: fix comment about "exceptional" bits

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>
---
 include/linux/radix-tree.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index cb4b7e8..f0fb327 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -35,7 +35,7 @@
  * 00 - data pointer
  * 01 - internal entry
  * 10 - exceptional entry
- * 11 - locked exceptional entry
+ * 11 - this bit combination is currently unused/reserved
  *
  * The internal entry may be a pointer to the next level in the tree, a
  * sibling entry, or an indicator that the entry in this slot has been moved
-- 
2.7.4