From 9f14d4f1f1045f161fd4db8a8e194b7825c2874a Mon Sep 17 00:00:00 2001
From: Matthew Wilcox <willy@infradead.org>
Date: Mon, 1 Oct 2018 14:54:59 -0400
Subject: [PATCH] xarray: Add xa_reserve and xa_release
This function reserves a slot in the XArray for users which need
to acquire multiple locks before storing their entry in the tree and
so cannot use a plain xa_store().
Signed-off-by: Matthew Wilcox <willy@infradead.org>
---
Documentation/core-api/xarray.rst | 6 ++++
include/linux/xarray.h | 34 ++++++++++++++++++++--
lib/test_xarray.c | 40 ++++++++++++++++++++++++++
lib/xarray.c | 47 +++++++++++++++++++++++++++++++
4 files changed, 125 insertions(+), 2 deletions(-)
diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst
index 4b101b4f3aa17..2b397da84bcdc 100644
--- a/Documentation/core-api/xarray.rst
+++ b/Documentation/core-api/xarray.rst
@@ -293,6 +293,12 @@ to :c:func:`xas_retry`, and retry the operation if it returns ``true``.
of this RCU period. You should restart the lookup from the head
of the array.
+ * - Zero
+ - :c:func:`xa_is_zero`
+ - Zero entries appear as ``NULL`` through the Normal API, but occupy
+ an entry in the XArray which can be used to reserve the index for
+ future use.
+
Other internal entries may be added in the future. As far as possible, they
will be handled by :c:func:`xas_retry`.
diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index deae17aa0ed7f..bed63d3cfea88 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -32,7 +32,8 @@
* The following internal entries have a special meaning:
*
* 0-62: Sibling entries
- * 256: Retry entry
+ * 256: Zero entry
+ * 257: Retry entry
*
* Errors are also represented as internal entries, but use the negative
* space (-4094 to -2). They're never stored in the slots array; only
@@ -277,6 +278,7 @@ void *xa_load(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_reserve(struct xarray *, unsigned long index, gfp_t);
bool xa_get_mark(struct xarray *, unsigned long index, xa_mark_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);
@@ -371,6 +373,20 @@ static inline int xa_insert(struct xarray *xa, unsigned long index,
return -EEXIST;
}
+/**
+ * xa_release() - Release a reserved entry.
+ * @xa: XArray.
+ * @index: Index of entry.
+ *
+ * After calling xa_reserve(), you can call this function to release the
+ * reservation. If the entry at @index has been stored to, this function
+ * will do nothing.
+ */
+static inline void xa_release(struct xarray *xa, unsigned long index)
+{
+ xa_cmpxchg(xa, index, NULL, NULL, 0);
+}
+
/**
* xa_for_each() - Iterate over a portion of an XArray.
* @xa: XArray.
@@ -658,7 +674,19 @@ static inline bool xa_is_sibling(const void *entry)
(entry < xa_mk_sibling(XA_CHUNK_SIZE - 1));
}
-#define XA_RETRY_ENTRY xa_mk_internal(256)
+#define XA_ZERO_ENTRY xa_mk_internal(256)
+#define XA_RETRY_ENTRY xa_mk_internal(257)
+
+/**
+ * xa_is_zero() - Is the entry a zero entry?
+ * @entry: Entry retrieved from the XArray
+ *
+ * Return: %true if the entry is a zero entry.
+ */
+static inline bool xa_is_zero(const void *entry)
+{
+ return unlikely(entry == XA_ZERO_ENTRY);
+}
/**
* xa_is_retry() - Is the entry a retry entry?
@@ -880,6 +908,8 @@ static inline void xas_reset(struct xa_state *xas)
*/
static inline bool xas_retry(struct xa_state *xas, const void *entry)
{
+ if (xa_is_zero(entry))
+ return true;
if (!xa_is_retry(entry))
return false;
xas_reset(xas);
diff --git a/lib/test_xarray.c b/lib/test_xarray.c
index 703370015d10d..6aafd411a5c32 100644
--- a/lib/test_xarray.c
+++ b/lib/test_xarray.c
@@ -259,6 +259,45 @@ static noinline void check_cmpxchg(struct xarray *xa)
XA_BUG_ON(xa, !xa_empty(xa));
}
+static noinline void check_reserve(struct xarray *xa)
+{
+ void *entry;
+ unsigned long index = 0;
+
+ /* An array with a reserved entry is not empty */
+ XA_BUG_ON(xa, !xa_empty(xa));
+ xa_reserve(xa, 12345678, GFP_KERNEL);
+ XA_BUG_ON(xa, xa_empty(xa));
+ XA_BUG_ON(xa, xa_load(xa, 12345678));
+ xa_release(xa, 12345678);
+ XA_BUG_ON(xa, !xa_empty(xa));
+
+ /* Releasing a used entry does nothing */
+ xa_reserve(xa, 12345678, GFP_KERNEL);
+ XA_BUG_ON(xa, xa_store_index(xa, 12345678, GFP_NOWAIT) != NULL);
+ xa_release(xa, 12345678);
+ xa_erase_index(xa, 12345678);
+ XA_BUG_ON(xa, !xa_empty(xa));
+
+ /* cmpxchg sees a reserved entry as NULL */
+ xa_reserve(xa, 12345678, GFP_KERNEL);
+ XA_BUG_ON(xa, xa_cmpxchg(xa, 12345678, NULL, xa_mk_value(12345678),
+ GFP_NOWAIT) != NULL);
+ xa_release(xa, 12345678);
+ xa_erase_index(xa, 12345678);
+ XA_BUG_ON(xa, !xa_empty(xa));
+
+ /* Can iterate through a reserved entry */
+ xa_store_index(xa, 5, GFP_KERNEL);
+ xa_reserve(xa, 6, GFP_KERNEL);
+ xa_store_index(xa, 7, GFP_KERNEL);
+
+ xa_for_each(xa, entry, index, ULONG_MAX, XA_PRESENT) {
+ XA_BUG_ON(xa, index != 5 && index != 7);
+ }
+ xa_destroy(xa);
+}
+
static noinline void check_xas_erase(struct xarray *xa)
{
XA_STATE(xas, xa, 0);
@@ -808,6 +847,7 @@ static int xarray_checks(void)
check_xa_shrink(&array);
check_xas_erase(&array);
check_cmpxchg(&array);
+ check_reserve(&array);
check_multi_store(&array);
check_find(&array);
check_destroy(&array);
diff --git a/lib/xarray.c b/lib/xarray.c
index ff37516fe8321..5464619142822 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1266,6 +1266,8 @@ void *xa_load(struct xarray *xa, unsigned long index)
rcu_read_lock();
do {
entry = xas_load(&xas);
+ if (xa_is_zero(entry))
+ entry = NULL;
} while (xas_retry(&xas, entry));
rcu_read_unlock();
@@ -1275,6 +1277,8 @@ EXPORT_SYMBOL(xa_load);
static void *xas_result(struct xa_state *xas, void *curr)
{
+ if (xa_is_zero(curr))
+ return NULL;
XA_NODE_BUG_ON(xas->xa_node, xa_is_internal(curr));
if (xas_error(xas))
curr = xas->xa_node;
@@ -1394,6 +1398,8 @@ void *xa_cmpxchg(struct xarray *xa, unsigned long index,
do {
xas_lock(&xas);
curr = xas_load(&xas);
+ if (curr == XA_ZERO_ENTRY)
+ curr = NULL;
if (curr == old)
xas_store(&xas, entry);
xas_unlock(&xas);
@@ -1430,6 +1436,8 @@ void *__xa_cmpxchg(struct xarray *xa, unsigned long index,
do {
curr = xas_load(&xas);
+ if (curr == XA_ZERO_ENTRY)
+ curr = NULL;
if (curr == old)
xas_store(&xas, entry);
} while (__xas_nomem(&xas, gfp));
@@ -1438,6 +1446,43 @@ void *__xa_cmpxchg(struct xarray *xa, unsigned long index,
}
EXPORT_SYMBOL(__xa_cmpxchg);
+/**
+ * xa_reserve() - Reserve this index in the XArray.
+ * @xa: XArray.
+ * @index: Index into array.
+ * @gfp: Memory allocation flags.
+ *
+ * Ensures there is somewhere to store an entry at @index in the array.
+ * If there is already something stored at @index, this function does
+ * nothing. If there was nothing there, the entry is marked as reserved.
+ * Loads from @index will continue to see a %NULL pointer until a
+ * subsequent store to @index.
+ *
+ * If you do not use the entry that you have reserved, call xa_release()
+ * or xa_erase() to free any unnecessary memory.
+ *
+ * Context: Process context. Takes and releases the xa_lock, IRQ or BH safe
+ * if specified in XArray flags. May sleep if the @gfp flags permit.
+ * Return: 0 if the reservation succeeded or -ENOMEM if it failed.
+ */
+int xa_reserve(struct xarray *xa, unsigned long index, gfp_t gfp)
+{
+ XA_STATE(xas, xa, index);
+ unsigned int lock_type = xa_lock_type(xa);
+ void *curr;
+
+ do {
+ xas_lock_type(&xas, lock_type);
+ curr = xas_load(&xas);
+ if (!curr)
+ xas_store(&xas, XA_ZERO_ENTRY);
+ xas_unlock_type(&xas, lock_type);
+ } while (xas_nomem(&xas, gfp));
+
+ return xas_error(&xas);
+}
+EXPORT_SYMBOL(xa_reserve);
+
/**
* __xa_set_mark() - Set this mark on this entry while locked.
* @xa: XArray.
@@ -1797,6 +1842,8 @@ void xa_dump_entry(const void *entry, unsigned long index, unsigned long shift)
pr_cont("retry (%ld)\n", xa_to_internal(entry));
else if (xa_is_sibling(entry))
pr_cont("sibling (slot %ld)\n", xa_to_sibling(entry));
+ else if (xa_is_zero(entry))
+ pr_cont("zero (%ld)\n", xa_to_internal(entry));
else
pr_cont("UNKNOWN ENTRY (%px)\n", entry);
}
--
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