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From:   Matthew Wilcox <willy@infradead.org>
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Cc:     Matthew Wilcox <mawilcox@microsoft.com>, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org,
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        David Howells <dhowells@redhat.com>
Subject: [PATCH v6 11/99] xarray: Add xa_cmpxchg and xa_insert
Date:   Wed, 17 Jan 2018 12:20:35 -0800
Message-Id: <20180117202203.19756-12-willy@infradead.org>
In-Reply-To: <20180117202203.19756-1-willy@infradead.org>
References: <20180117202203.19756-1-willy@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Matthew Wilcox <mawilcox@microsoft.com>

Like cmpxchg(), xa_cmpxchg will only store to the index if the current
entry matches the old entry.  It returns the current entry, which is
usually more useful than the errno returned by radix_tree_insert().
For the users who really only want the errno, the xa_insert() wrapper
provides a more convenient calling convention.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
---
 include/linux/xarray.h                 | 56 ++++++++++++++++++++++++++++
 lib/xarray.c                           | 68 ++++++++++++++++++++++++++++++++++
 tools/testing/radix-tree/xarray-test.c | 10 +++++
 3 files changed, 134 insertions(+)

diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index 139b1c1fd022..fc9ab3b13e60 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -210,6 +210,8 @@ struct xarray {
 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);
+void *xa_cmpxchg(struct xarray *, unsigned long index,
+			void *old, void *entry, gfp_t);
 bool xa_get_tag(struct xarray *, unsigned long index, xa_tag_t);
 void xa_set_tag(struct xarray *, unsigned long index, xa_tag_t);
 void xa_clear_tag(struct xarray *, unsigned long index, xa_tag_t);
@@ -264,6 +266,32 @@ static inline bool xa_tagged(const struct xarray *xa, xa_tag_t tag)
 	return xa->xa_flags & XA_FLAGS_TAG(tag);
 }
 
+/**
+ * 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.
+ *
+ * Return: -EEXIST if another entry was present, 0 if the store succeeded,
+ * or another negative errno if a different error happened (eg -ENOMEM).
+ */
+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;
+}
+
 #define xa_trylock(xa)		spin_trylock(&(xa)->xa_lock)
 #define xa_lock(xa)		spin_lock(&(xa)->xa_lock)
 #define xa_unlock(xa)		spin_unlock(&(xa)->xa_lock)
@@ -283,9 +311,37 @@ static inline bool xa_tagged(const struct xarray *xa, xa_tag_t tag)
  */
 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_set_tag(struct xarray *, unsigned long index, xa_tag_t);
 void __xa_clear_tag(struct xarray *, unsigned long index, xa_tag_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.
+ *
+ * Return: -EEXIST if another entry was present, 0 if the store succeeded,
+ * or another negative errno if a different error happened (eg -ENOMEM).
+ */
+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;
+}
+
 /* Everything below here is the Advanced API.  Proceed with caution. */
 
 /*
diff --git a/lib/xarray.c b/lib/xarray.c
index 45b70e622bf1..d925a98fb9b8 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -928,6 +928,74 @@ void *__xa_store(struct xarray *xa, unsigned long index, void *entry, gfp_t gfp)
 }
 EXPORT_SYMBOL(__xa_store);
 
+/**
+ * xa_cmpxchg() - 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.
+ *
+ * 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.
+ *
+ * Return: The old value at this index or xa_err() if an error happened.
+ */
+void *xa_cmpxchg(struct xarray *xa, unsigned long index,
+			void *old, void *entry, gfp_t gfp)
+{
+	XA_STATE(xas, xa, index);
+	void *curr;
+
+	if (WARN_ON_ONCE(xa_is_internal(entry)))
+		return XA_ERROR(-EINVAL);
+
+	do {
+		xas_lock(&xas);
+		curr = xas_load(&xas);
+		if (curr == old)
+			xas_store(&xas, entry);
+		xas_unlock(&xas);
+	} while (xas_nomem(&xas, gfp));
+
+	return xas_result(&xas, curr);
+}
+EXPORT_SYMBOL(xa_cmpxchg);
+
+/**
+ * __xa_cmpxchg() - Store this entry in the XArray.
+ * @xa: XArray.
+ * @index: Index into array.
+ * @old: Old value to test against.
+ * @entry: New entry.
+ * @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.
+ *
+ *
+ * Return: The old entry at this index or xa_err() if an error happened.
+ */
+void *__xa_cmpxchg(struct xarray *xa, unsigned long index,
+			void *old, void *entry, gfp_t gfp)
+{
+	XA_STATE(xas, xa, index);
+	void *curr;
+
+	if (WARN_ON_ONCE(xa_is_internal(entry)))
+		return XA_ERROR(-EINVAL);
+
+	do {
+		curr = xas_load(&xas);
+		if (curr == old)
+			xas_store(&xas, entry);
+	} while (__xas_nomem(&xas, gfp));
+
+	return xas_result(&xas, curr);
+}
+EXPORT_SYMBOL(__xa_cmpxchg);
+
 /**
  * __xa_set_tag() - Set this tag on this entry while locked.
  * @xa: XArray.
diff --git a/tools/testing/radix-tree/xarray-test.c b/tools/testing/radix-tree/xarray-test.c
index 5defd0b9f85c..d6a969d999d9 100644
--- a/tools/testing/radix-tree/xarray-test.c
+++ b/tools/testing/radix-tree/xarray-test.c
@@ -84,6 +84,15 @@ void check_xa_shrink(struct xarray *xa)
 	assert(xa_load(xa, 0) == xa_mk_value(0));
 }
 
+void check_cmpxchg(struct xarray *xa)
+{
+	assert(xa_empty(xa));
+	assert(!xa_store(xa, 12345678, xa_mk_value(12345678), GFP_KERNEL));
+	assert(!xa_cmpxchg(xa, 5, xa_mk_value(5), NULL, GFP_KERNEL));
+	assert(xa_erase(xa, 12345678) == xa_mk_value(12345678));
+	assert(xa_empty(xa));
+}
+
 void check_multi_store(struct xarray *xa)
 {
 	unsigned long i, j, k;
@@ -149,6 +158,7 @@ void xarray_checks(void)
 	check_xa_shrink(&array);
 	item_kill_tree(&array);
 
+	check_cmpxchg(&array);
 	check_multi_store(&array);
 	item_kill_tree(&array);
 }
-- 
2.15.1