rust: xarray: add insert and reserve

Add `Guard::{insert,reserve}` and `Guard::{insert,reserve}_limit`, which
are akin to `__xa_{alloc,insert}` in C.

Note that unlike `xa_reserve` which only ensures that memory is
allocated, the semantics of `Reservation` are stricter and require
precise management of the reservation. Indices which have been reserved
can still be overwritten with `Guard::store`, which allows for C-like
semantics if desired.

`__xa_cmpxchg_raw` is exported to facilitate the semantics described
above.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>

Author: tamird

include/linux/xarray.h

@@ -563,6 +563,8 @@ 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);
+void *__xa_cmpxchg_raw(struct xarray *, unsigned long index, void *old,
+		void *entry, gfp_t);
 int __must_check __xa_insert(struct xarray *, unsigned long index,
 		void *entry, gfp_t);
 int __must_check __xa_alloc(struct xarray *, u32 *id, void *entry,

lib/xarray.c

@@ -1738,9 +1738,6 @@ void *xa_store(struct xarray *xa, unsigned long index, void *entry, gfp_t gfp)
 }
 EXPORT_SYMBOL(xa_store);
 
-static inline void *__xa_cmpxchg_raw(struct xarray *xa, unsigned long index,
-			void *old, void *entry, gfp_t gfp);
-
 /**
  * __xa_cmpxchg() - Conditionally replace an entry in the XArray.
  * @xa: XArray.
@@ -1767,7 +1764,29 @@ void *__xa_cmpxchg(struct xarray *xa, unsigned long index,
 }
 EXPORT_SYMBOL(__xa_cmpxchg);
 
-static inline void *__xa_cmpxchg_raw(struct xarray *xa, unsigned long index,
+/**
+ * __xa_cmpxchg_raw() - Conditionally replace an entry in the XArray.
+ * @xa: XArray.
+ * @index: Index into array.
+ * @old: Old value to test against.
+ * @entry: New value to place in array.
+ * @gfp: Memory allocation flags.
+ *
+ * You must already be holding the xa_lock when calling this function.
+ * It will drop the lock if needed to allocate memory, and then reacquire
+ * it afterwards.
+ *
+ * If the entry at @index is the same as @old, replace it with @entry.
+ * If the return value is equal to @old, then the exchange was successful.
+ *
+ * This function is the same as __xa_cmpxchg() except that it does not coerce
+ * XA_ZERO_ENTRY to NULL on egress.
+ *
+ * Context: Any context.  Expects xa_lock to be held on entry.  May
+ * release and reacquire xa_lock if @gfp flags permit.
+ * Return: The old value at this index or xa_err() if an error happened.
+ */
+void *__xa_cmpxchg_raw(struct xarray *xa, unsigned long index,
 			void *old, void *entry, gfp_t gfp)
 {
 	XA_STATE(xas, xa, index);
@@ -1787,6 +1806,7 @@ static inline void *__xa_cmpxchg_raw(struct xarray *xa, unsigned long index,
 
 	return xas_result(&xas, curr);
 }
+EXPORT_SYMBOL(__xa_cmpxchg_raw);
 
 /**
  * __xa_insert() - Store this entry in the XArray if no entry is present.

rust/helpers/xarray.c

@@ -2,6 +2,11 @@
 
 #include <linux/xarray.h>
 
+void *rust_helper_xa_zero_entry(void)
+{
+	return XA_ZERO_ENTRY;
+}
+
 int rust_helper_xa_err(void *entry)
 {
 	return xa_err(entry);