xarray: Track free entries in an XArray

Add the optional ability to track which entries in an XArray are free
and provide xa_alloc() to replace most of the functionality of the IDR.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

Documentation/core-api/xarray.rst

@@ -103,12 +103,25 @@ Finally, you can remove all entries from an XArray by calling
 to free the entries first.  You can do this by iterating over all present
 entries in the XArray using the :c:func:`xa_for_each` iterator.
 
+ID assignment
+-------------
+
+You can call :c:func:`xa_alloc` to store the entry at any 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.  Unlike :c:func:`xa_store`, allocating
+a ``NULL`` pointer does not delete an entry.  Instead it reserves an
+entry like :c:func:`xa_reserve` and you can release it using either
+:c:func:`xa_erase` or :c:func:`xa_release`.  To use ID assignment, the
+XArray must be defined with :c:func:`DEFINE_XARRAY_ALLOC`, or initialised
+by passing ``XA_FLAGS_ALLOC`` to :c:func:`xa_init_flags`,
+
 Memory allocation
 -----------------
 
-The :c:func:`xa_store`, :c:func:`xa_cmpxchg`, :c:func:`xa_reserve`
-and :c:func:`xa_insert` functions take a gfp_t parameter in case
-the XArray needs to allocate memory to store this entry.
+The :c:func:`xa_store`, :c:func:`xa_cmpxchg`, :c:func:`xa_alloc`,
+:c:func:`xa_reserve` and :c:func:`xa_insert` functions take a gfp_t
+parameter in case the XArray needs to allocate memory to store this entry.
 If the entry is being deleted, no memory allocation needs to be performed,
 and the GFP flags specified will be ignored.
 
@@ -143,6 +156,9 @@ Takes xa_lock internally:
  * :c:func:`xa_erase_bh`
  * :c:func:`xa_erase_irq`
  * :c:func:`xa_cmpxchg`
+ * :c:func:`xa_alloc`
+ * :c:func:`xa_alloc_bh`
+ * :c:func:`xa_alloc_irq`
  * :c:func:`xa_destroy`
  * :c:func:`xa_set_mark`
  * :c:func:`xa_clear_mark`
@@ -152,6 +168,7 @@ Assumes xa_lock held on entry:
  * :c:func:`__xa_insert`
  * :c:func:`__xa_erase`
  * :c:func:`__xa_cmpxchg`
+ * :c:func:`__xa_alloc`
  * :c:func:`__xa_set_mark`
  * :c:func:`__xa_clear_mark`
 

include/linux/xarray.h

@@ -205,6 +205,7 @@ typedef unsigned __bitwise xa_mark_t;
 #define XA_MARK_2		((__force xa_mark_t)2U)
 #define XA_PRESENT		((__force xa_mark_t)8U)
 #define XA_MARK_MAX		XA_MARK_2
+#define XA_FREE_MARK		XA_MARK_0
 
 enum xa_lock_type {
 	XA_LOCK_IRQ = 1,
@@ -217,9 +218,12 @@ enum xa_lock_type {
  */
 #define XA_FLAGS_LOCK_IRQ	((__force gfp_t)XA_LOCK_IRQ)
 #define XA_FLAGS_LOCK_BH	((__force gfp_t)XA_LOCK_BH)
+#define XA_FLAGS_TRACK_FREE	((__force gfp_t)4U)
 #define XA_FLAGS_MARK(mark)	((__force gfp_t)((1U << __GFP_BITS_SHIFT) << \
 						(__force unsigned)(mark)))
 
+#define XA_FLAGS_ALLOC	(XA_FLAGS_TRACK_FREE | XA_FLAGS_MARK(XA_FREE_MARK))
+
 /**
  * struct xarray - The anchor of the XArray.
  * @xa_lock: Lock that protects the contents of the XArray.
@@ -273,6 +277,15 @@ struct xarray {
  */
 #define DEFINE_XARRAY(name) DEFINE_XARRAY_FLAGS(name, 0)
 
+/**
+ * DEFINE_XARRAY_ALLOC() - Define an XArray which can allocate IDs.
+ * @name: A string that names your XArray.
+ *
+ * This is intended for file scope definitions of allocating XArrays.
+ * See also DEFINE_XARRAY().
+ */
+#define DEFINE_XARRAY_ALLOC(name) DEFINE_XARRAY_FLAGS(name, XA_FLAGS_ALLOC)
+
 void xa_init_flags(struct xarray *, gfp_t flags);
 void *xa_load(struct xarray *, unsigned long index);
 void *xa_store(struct xarray *, unsigned long index, void *entry, gfp_t);
@@ -439,6 +452,7 @@ void *__xa_erase(struct xarray *, unsigned long index);
 void *__xa_store(struct xarray *, unsigned long index, void *entry, gfp_t);
 void *__xa_cmpxchg(struct xarray *, unsigned long index, void *old,
 		void *entry, gfp_t);
+int __xa_alloc(struct xarray *, u32 *id, u32 max, void *entry, gfp_t);
 void __xa_set_mark(struct xarray *, unsigned long index, xa_mark_t);
 void __xa_clear_mark(struct xarray *, unsigned long index, xa_mark_t);
 
@@ -518,6 +532,93 @@ static inline void *xa_erase_irq(struct xarray *xa, unsigned long index)
 	return entry;
 }
 
+/**
+ * xa_alloc() - Find somewhere to store this entry in the XArray.
+ * @xa: XArray.
+ * @id: Pointer to ID.
+ * @max: Maximum ID to allocate (inclusive).
+ * @entry: New entry.
+ * @gfp: Memory allocation flags.
+ *
+ * Allocates an unused ID in the range specified by @id and @max.
+ * Updates the @id pointer with the index, then stores the entry at that
+ * index.  A concurrent lookup will not see an uninitialised @id.
+ *
+ * Context: Process context.  Takes and releases the xa_lock.  May sleep if
+ * the @gfp flags permit.
+ * Return: 0 on success, -ENOMEM if memory allocation fails or -ENOSPC if
+ * there is no more space in the XArray.
+ */
+static inline int xa_alloc(struct xarray *xa, u32 *id, u32 max, void *entry,
+		gfp_t gfp)
+{
+	int err;
+
+	xa_lock(xa);
+	err = __xa_alloc(xa, id, max, entry, gfp);
+	xa_unlock(xa);
+
+	return err;
+}
+
+/**
+ * xa_alloc_bh() - Find somewhere to store this entry in the XArray.
+ * @xa: XArray.
+ * @id: Pointer to ID.
+ * @max: Maximum ID to allocate (inclusive).
+ * @entry: New entry.
+ * @gfp: Memory allocation flags.
+ *
+ * Allocates an unused ID in the range specified by @id and @max.
+ * Updates the @id pointer with the index, then stores the entry at that
+ * index.  A concurrent lookup will not see an uninitialised @id.
+ *
+ * Context: Process context.  Takes and releases the xa_lock while
+ * disabling softirqs.  May sleep if the @gfp flags permit.
+ * Return: 0 on success, -ENOMEM if memory allocation fails or -ENOSPC if
+ * there is no more space in the XArray.
+ */
+static inline int xa_alloc_bh(struct xarray *xa, u32 *id, u32 max, void *entry,
+		gfp_t gfp)
+{
+	int err;
+
+	xa_lock_bh(xa);
+	err = __xa_alloc(xa, id, max, entry, gfp);
+	xa_unlock_bh(xa);
+
+	return err;
+}
+
+/**
+ * xa_alloc_irq() - Find somewhere to store this entry in the XArray.
+ * @xa: XArray.
+ * @id: Pointer to ID.
+ * @max: Maximum ID to allocate (inclusive).
+ * @entry: New entry.
+ * @gfp: Memory allocation flags.
+ *
+ * Allocates an unused ID in the range specified by @id and @max.
+ * Updates the @id pointer with the index, then stores the entry at that
+ * index.  A concurrent lookup will not see an uninitialised @id.
+ *
+ * Context: Process context.  Takes and releases the xa_lock while
+ * disabling interrupts.  May sleep if the @gfp flags permit.
+ * Return: 0 on success, -ENOMEM if memory allocation fails or -ENOSPC if
+ * there is no more space in the XArray.
+ */
+static inline int xa_alloc_irq(struct xarray *xa, u32 *id, u32 max, void *entry,
+		gfp_t gfp)
+{
+	int err;
+
+	xa_lock_irq(xa);
+	err = __xa_alloc(xa, id, max, entry, gfp);
+	xa_unlock_irq(xa);
+
+	return err;
+}
+
 /* Everything below here is the Advanced API.  Proceed with caution. */
 
 /*

lib/test_xarray.c

@@ -33,6 +33,15 @@ static void *xa_store_index(struct xarray *xa, unsigned long index, gfp_t gfp)
 	return xa_store(xa, index, xa_mk_value(index & LONG_MAX), gfp);
 }
 
+static void xa_alloc_index(struct xarray *xa, unsigned long index, gfp_t gfp)
+{
+	u32 id = 0;
+
+	XA_BUG_ON(xa, xa_alloc(xa, &id, UINT_MAX, xa_mk_value(index & LONG_MAX),
+				gfp) != 0);
+	XA_BUG_ON(xa, id != index);
+}
+
 static void xa_erase_index(struct xarray *xa, unsigned long index)
 {
 	XA_BUG_ON(xa, xa_erase(xa, index) != xa_mk_value(index & LONG_MAX));
@@ -404,6 +413,57 @@ static noinline void check_multi_store(struct xarray *xa)
 #endif
 }
 
+static DEFINE_XARRAY_ALLOC(xa0);
+
+static noinline void check_xa_alloc(void)
+{
+	int i;
+	u32 id;
+
+	/* An empty array should assign 0 to the first alloc */
+	xa_alloc_index(&xa0, 0, GFP_KERNEL);
+
+	/* Erasing it should make the array empty again */
+	xa_erase_index(&xa0, 0);
+	XA_BUG_ON(&xa0, !xa_empty(&xa0));
+
+	/* And it should assign 0 again */
+	xa_alloc_index(&xa0, 0, GFP_KERNEL);
+
+	/* The next assigned ID should be 1 */
+	xa_alloc_index(&xa0, 1, GFP_KERNEL);
+	xa_erase_index(&xa0, 1);
+
+	/* Storing a value should mark it used */
+	xa_store_index(&xa0, 1, GFP_KERNEL);
+	xa_alloc_index(&xa0, 2, GFP_KERNEL);
+
+	/* If we then erase 0, it should be free */
+	xa_erase_index(&xa0, 0);
+	xa_alloc_index(&xa0, 0, GFP_KERNEL);
+
+	xa_erase_index(&xa0, 1);
+	xa_erase_index(&xa0, 2);
+
+	for (i = 1; i < 5000; i++) {
+		xa_alloc_index(&xa0, i, GFP_KERNEL);
+	}
+
+	xa_destroy(&xa0);
+
+	id = 0xfffffffeU;
+	XA_BUG_ON(&xa0, xa_alloc(&xa0, &id, UINT_MAX, xa_mk_value(0),
+				GFP_KERNEL) != 0);
+	XA_BUG_ON(&xa0, id != 0xfffffffeU);
+	XA_BUG_ON(&xa0, xa_alloc(&xa0, &id, UINT_MAX, xa_mk_value(0),
+				GFP_KERNEL) != 0);
+	XA_BUG_ON(&xa0, id != 0xffffffffU);
+	XA_BUG_ON(&xa0, xa_alloc(&xa0, &id, UINT_MAX, xa_mk_value(0),
+				GFP_KERNEL) != -ENOSPC);
+	XA_BUG_ON(&xa0, id != 0xffffffffU);
+	xa_destroy(&xa0);
+}
+
 static noinline void __check_store_iter(struct xarray *xa, unsigned long start,
 			unsigned int order, unsigned int present)
 {
@@ -849,6 +909,7 @@ static int xarray_checks(void)
 	check_cmpxchg(&array);
 	check_reserve(&array);
 	check_multi_store(&array);
+	check_xa_alloc();
 	check_find(&array);
 	check_destroy(&array);
 	check_move(&array);