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 3bcadd6fa6 ("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.
Link: http://lkml.kernel.org/r/1468997731-2155-1-git-send-email-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
a23216a2f1