XArray: Honour reserved entries in xa_insert

xa_insert() should treat reserved entries as occupied, not as available.
Also, it should treat requests to insert a NULL pointer as a request
to reserve the slot.  Add xa_insert_bh() and xa_insert_irq() for
completeness.
Signed-off-by: Matthew Wilcox <willy@infradead.org>

Documentation/core-api/xarray.rst

@@ -108,12 +108,13 @@ some, but not all of the other indices changing.
 
 Sometimes you need to ensure that a subsequent call to :c:func:`xa_store`
 will not need to allocate memory.  The :c:func:`xa_reserve` function
-will store a reserved entry at the indicated index.  Users of the normal
-API will see this entry as containing ``NULL``.  If you do not need to
-use the reserved entry, you can call :c:func:`xa_release` to remove the
-unused entry.  If another user has stored to the entry in the meantime,
-:c:func:`xa_release` will do nothing; if instead you want the entry to
-become ``NULL``, you should use :c:func:`xa_erase`.
+will store a reserved entry at the indicated index.  Users of the
+normal API will see this entry as containing ``NULL``.  If you do
+not need to use the reserved entry, you can call :c:func:`xa_release`
+to remove the unused entry.  If another user has stored to the entry
+in the meantime, :c:func:`xa_release` will do nothing; if instead you
+want the entry to become ``NULL``, you should use :c:func:`xa_erase`.
+Using :c:func:`xa_insert` on a reserved entry will fail.
 
 If all entries in the array are ``NULL``, the :c:func:`xa_empty` function
 will return ``true``.
@@ -183,6 +184,8 @@ Takes xa_lock internally:
  * :c:func:`xa_store_bh`
  * :c:func:`xa_store_irq`
  * :c:func:`xa_insert`
+ * :c:func:`xa_insert_bh`
+ * :c:func:`xa_insert_irq`
  * :c:func:`xa_erase`
  * :c:func:`xa_erase_bh`
  * :c:func:`xa_erase_irq`

include/linux/xarray.h

@@ -463,39 +463,12 @@ 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_insert(struct xarray *, unsigned long index, void *entry, gfp_t);
 int __xa_alloc(struct xarray *, u32 *id, u32 max, void *entry, gfp_t);
 int __xa_reserve(struct xarray *, unsigned long index, 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);
 
-/**
- * __xa_insert() - Store this entry in the XArray unless another entry is
- *			already present.
- * @xa: XArray.
- * @index: Index into array.
- * @entry: New entry.
- * @gfp: Memory allocation flags.
- *
- * If you would rather see the existing entry in the array, use __xa_cmpxchg().
- * This function is for users who don't care what the entry is, only that
- * one is present.
- *
- * Context: Any context.  Expects xa_lock to be held on entry.  May
- *	    release and reacquire xa_lock if the @gfp flags permit.
- * Return: 0 if the store succeeded.  -EEXIST if another entry was present.
- * -ENOMEM if memory could not be allocated.
- */
-static inline int __xa_insert(struct xarray *xa, unsigned long index,
-		void *entry, gfp_t gfp)
-{
-	void *curr = __xa_cmpxchg(xa, index, NULL, entry, gfp);
-	if (!curr)
-		return 0;
-	if (xa_is_err(curr))
-		return xa_err(curr);
-	return -EEXIST;
-}
-
 /**
  * xa_store_bh() - Store this entry in the XArray.
  * @xa: XArray.
@@ -685,24 +658,83 @@ static inline void *xa_cmpxchg_irq(struct xarray *xa, unsigned long index,
  * @entry: New entry.
  * @gfp: Memory allocation flags.
  *
- * If you would rather see the existing entry in the array, use xa_cmpxchg().
- * This function is for users who don't care what the entry is, only that
- * one is present.
+ * Inserting a NULL entry will store a reserved entry (like xa_reserve())
+ * if no entry is present.  Inserting will fail if a reserved entry is
+ * present, even though loading from this index will return NULL.
  *
- * Context: Process context.  Takes and releases the xa_lock.
- *	    May sleep if the @gfp flags permit.
+ * Context: Any context.  Takes and releases the xa_lock.  May sleep if
+ * the @gfp flags permit.
  * Return: 0 if the store succeeded.  -EEXIST if another entry was present.
  * -ENOMEM if memory could not be allocated.
  */
 static inline int xa_insert(struct xarray *xa, unsigned long index,
 		void *entry, gfp_t gfp)
 {
-	void *curr = xa_cmpxchg(xa, index, NULL, entry, gfp);
-	if (!curr)
-		return 0;
-	if (xa_is_err(curr))
-		return xa_err(curr);
-	return -EEXIST;
+	int err;
+
+	xa_lock(xa);
+	err = __xa_insert(xa, index, entry, gfp);
+	xa_unlock(xa);
+
+	return err;
+}
+
+/**
+ * xa_insert_bh() - Store this entry in the XArray unless another entry is
+ *			already present.
+ * @xa: XArray.
+ * @index: Index into array.
+ * @entry: New entry.
+ * @gfp: Memory allocation flags.
+ *
+ * Inserting a NULL entry will store a reserved entry (like xa_reserve())
+ * if no entry is present.  Inserting will fail if a reserved entry is
+ * present, even though loading from this index will return NULL.
+ *
+ * Context: Any context.  Takes and releases the xa_lock while
+ * disabling softirqs.  May sleep if the @gfp flags permit.
+ * Return: 0 if the store succeeded.  -EEXIST if another entry was present.
+ * -ENOMEM if memory could not be allocated.
+ */
+static inline int xa_insert_bh(struct xarray *xa, unsigned long index,
+		void *entry, gfp_t gfp)
+{
+	int err;
+
+	xa_lock_bh(xa);
+	err = __xa_insert(xa, index, entry, gfp);
+	xa_unlock_bh(xa);
+
+	return err;
+}
+
+/**
+ * xa_insert_irq() - Store this entry in the XArray unless another entry is
+ *			already present.
+ * @xa: XArray.
+ * @index: Index into array.
+ * @entry: New entry.
+ * @gfp: Memory allocation flags.
+ *
+ * Inserting a NULL entry will store a reserved entry (like xa_reserve())
+ * if no entry is present.  Inserting will fail if a reserved entry is
+ * present, even though loading from this index will return NULL.
+ *
+ * Context: Process context.  Takes and releases the xa_lock while
+ * disabling interrupts.  May sleep if the @gfp flags permit.
+ * Return: 0 if the store succeeded.  -EEXIST if another entry was present.
+ * -ENOMEM if memory could not be allocated.
+ */
+static inline int xa_insert_irq(struct xarray *xa, unsigned long index,
+		void *entry, gfp_t gfp)
+{
+	int err;
+
+	xa_lock_irq(xa);
+	err = __xa_insert(xa, index, entry, gfp);
+	xa_unlock_irq(xa);
+
+	return err;
 }
 
 /**

lib/test_xarray.c

@@ -382,10 +382,12 @@ static noinline void check_reserve(struct xarray *xa)
 	xa_erase_index(xa, 12345678);
 	XA_BUG_ON(xa, !xa_empty(xa));
 
-	/* And so does xa_insert */
+	/* But xa_insert does not */
 	xa_reserve(xa, 12345678, GFP_KERNEL);
-	XA_BUG_ON(xa, xa_insert(xa, 12345678, xa_mk_value(12345678), 0) != 0);
-	xa_erase_index(xa, 12345678);
+	XA_BUG_ON(xa, xa_insert(xa, 12345678, xa_mk_value(12345678), 0) !=
+			-EEXIST);
+	XA_BUG_ON(xa, xa_empty(xa));
+	XA_BUG_ON(xa, xa_erase(xa, 12345678) != NULL);
 	XA_BUG_ON(xa, !xa_empty(xa));
 
 	/* Can iterate through a reserved entry */

lib/xarray.c

@@ -1439,6 +1439,47 @@ void *__xa_cmpxchg(struct xarray *xa, unsigned long index,
 }
 EXPORT_SYMBOL(__xa_cmpxchg);
 
+/**
+ * __xa_insert() - Store this entry in the XArray if no entry is present.
+ * @xa: XArray.
+ * @index: Index into array.
+ * @entry: New entry.
+ * @gfp: Memory allocation flags.
+ *
+ * Inserting a NULL entry will store a reserved entry (like xa_reserve())
+ * if no entry is present.  Inserting will fail if a reserved entry is
+ * present, even though loading from this index will return NULL.
+ *
+ * Context: Any context.  Expects xa_lock to be held on entry.  May
+ * release and reacquire xa_lock if @gfp flags permit.
+ * Return: 0 if the store succeeded.  -EEXIST if another entry was present.
+ * -ENOMEM if memory could not be allocated.
+ */
+int __xa_insert(struct xarray *xa, unsigned long index, void *entry, gfp_t gfp)
+{
+	XA_STATE(xas, xa, index);
+	void *curr;
+
+	if (WARN_ON_ONCE(xa_is_advanced(entry)))
+		return -EINVAL;
+	if (!entry)
+		entry = XA_ZERO_ENTRY;
+
+	do {
+		curr = xas_load(&xas);
+		if (!curr) {
+			xas_store(&xas, entry);
+			if (xa_track_free(xa))
+				xas_clear_mark(&xas, XA_FREE_MARK);
+		} else {
+			xas_set_err(&xas, -EEXIST);
+		}
+	} while (__xas_nomem(&xas, gfp));
+
+	return xas_error(&xas);
+}
+EXPORT_SYMBOL(__xa_insert);
+
 /**
  * __xa_reserve() - Reserve this index in the XArray.
  * @xa: XArray.