linux-kernel - [PATCH 1/2] xarray: extract xa_zero_to

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Message-Id: <20241112-xarray-insert-cmpxchg-v1-1-dc2bdd8c4136@gmail.com>
Date: Tue, 12 Nov 2024 14:25:36 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>, 
 Matthew Wilcox <willy@...radead.org>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Alice Ryhl <aliceryhl@...gle.com>, Andreas Hindborg <a.hindborg@...nel.org>, 
 Tamir Duberstein <tamird@...il.com>
Subject: [PATCH 1/2] xarray: extract xa_zero_to_null

Reduce code duplication by extracting a static inline function that
returns its argument if it is non-zero and NULL otherwise.

This changes xas_result to check for errors before checking for zero but
this cannot change the behavior of existing callers:
- __xa_erase: passes the result of xas_store(_, NULL) which cannot fail.
- __xa_store: passes the result of xas_store(_, entry) which may fail.
  xas_store calls xas_create when entry is not NULL which returns NULL
  on error, which is immediately checked. This should not change
  observable behavior.
- __xa_cmpxchg: passes the result of xas_load(_) which might be zero.
  This would previously return NULL regardless of the outcome of
  xas_store but xas_store cannot fail if xas_load returns zero
  because there is no need to allocate memory.
- xa_store_range: same as __xa_erase.

Signed-off-by: Tamir Duberstein <tamird@...il.com>
---
 lib/xarray.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/lib/xarray.c b/lib/xarray.c
index 32d4bac8c94ca13e11f350c6bcfcacc2040d0359..1b8305bffbff07adb80334fc83b5dc8e40ba2f50 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -435,6 +435,11 @@ static unsigned long max_index(void *entry)
 	return (XA_CHUNK_SIZE << xa_to_node(entry)->shift) - 1;
 }
 
+static inline void *xa_zero_to_null(void *entry)
+{
+	return xa_is_zero(entry) ? NULL : entry;
+}
+
 static void xas_shrink(struct xa_state *xas)
 {
 	struct xarray *xa = xas->xa;
@@ -451,8 +456,8 @@ static void xas_shrink(struct xa_state *xas)
 			break;
 		if (!xa_is_node(entry) && node->shift)
 			break;
-		if (xa_is_zero(entry) && xa_zero_busy(xa))
-			entry = NULL;
+		if (xa_zero_busy(xa))
+			entry = xa_zero_to_null(entry);
 		xas->xa_node = XAS_BOUNDS;
 
 		RCU_INIT_POINTER(xa->xa_head, entry);
@@ -1474,9 +1479,7 @@ void *xa_load(struct xarray *xa, unsigned long index)
 
 	rcu_read_lock();
 	do {
-		entry = xas_load(&xas);
-		if (xa_is_zero(entry))
-			entry = NULL;
+		entry = xa_zero_to_null(xas_load(&xas));
 	} while (xas_retry(&xas, entry));
 	rcu_read_unlock();
 
@@ -1486,11 +1489,9 @@ EXPORT_SYMBOL(xa_load);
 
 static void *xas_result(struct xa_state *xas, void *curr)
 {
-	if (xa_is_zero(curr))
-		return NULL;
 	if (xas_error(xas))
 		curr = xas->xa_node;
-	return curr;
+	return xa_zero_to_null(curr);
 }
 
 /**

-- 
2.47.0