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authorMatthew Wilcox <willy@infradead.org>2018-10-26 14:43:22 -0400
committerMatthew Wilcox <willy@infradead.org>2019-02-06 13:13:24 -0500
commit3ccaf57a6a63ad171a951dcaddffc453b2414c7b (patch)
treefc2202432a5b50ee5507a2e240b439b2993c2c3f /Documentation/core-api
parentfd9dc93e36231fb6d520e0edd467058fad4fd12d (diff)
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XArray: Add support for 1s-based allocation
A lot of places want to allocate IDs starting at 1 instead of 0. While the xa_alloc() API supports this, it's not very efficient if lots of IDs are allocated, due to having to walk down to the bottom of the tree to see if ID 1 is available, then all the way over to the next non-allocated ID. This method marks ID 0 as being occupied which wastes one slot in the XArray, but preserves xa_empty() as working. Signed-off-by: Matthew Wilcox <willy@infradead.org>
Diffstat (limited to 'Documentation/core-api')
-rw-r--r--Documentation/core-api/xarray.rst10
1 files changed, 7 insertions, 3 deletions
diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst
index 42bb1a62650f..e90c4925cd37 100644
--- a/Documentation/core-api/xarray.rst
+++ b/Documentation/core-api/xarray.rst
@@ -131,17 +131,21 @@ If you use :c:func:`DEFINE_XARRAY_ALLOC` to define the XArray, or
initialise it by passing ``XA_FLAGS_ALLOC`` to :c:func:`xa_init_flags`,
the XArray changes to track whether entries are in use or not.
-You can call :c:func:`xa_alloc` to store the entry at any unused index
+You can call :c:func:`xa_alloc` to store the entry at an unused index
in the XArray. If you need to modify the array from interrupt context,
you can use :c:func:`xa_alloc_bh` or :c:func:`xa_alloc_irq` to disable
interrupts while allocating the ID.
-Using :c:func:`xa_store`, :c:func:`xa_cmpxchg` or :c:func:`xa_insert`
-will mark the entry as being allocated. Unlike a normal XArray, storing
+Using :c:func:`xa_store`, :c:func:`xa_cmpxchg` or :c:func:`xa_insert` will
+also mark the entry as being allocated. Unlike a normal XArray, storing
``NULL`` will mark the entry as being in use, like :c:func:`xa_reserve`.
To free an entry, use :c:func:`xa_erase` (or :c:func:`xa_release` if
you only want to free the entry if it's ``NULL``).
+By default, the lowest free entry is allocated starting from 0. If you
+want to allocate entries starting at 1, it is more efficient to use
+:c:func:`DEFINE_XARRAY_ALLOC1` or ``XA_FLAGS_ALLOC1``.
+
You cannot use ``XA_MARK_0`` with an allocating XArray as this mark
is used to track whether an entry is free or not. The other marks are
available for your use.