From: Olivier Houchard <ohouchard@haproxy.com>
Date: Tue, 25 Mar 2025 16:17:56 +0000 (+0000)
Subject: MINOR: mt_list: Implement mt_list_try_lock_prev().
X-Git-Tag: v3.2-dev9~28
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=17059098e7ba74d5b15964e3f25e4d94251fcce2;p=thirdparty%2Fhaproxy.git

MINOR: mt_list: Implement mt_list_try_lock_prev().

Implement mt_list_try_lock_prev(), that does the same thing
as mt_list_lock_prev(), exceot if the list is locked, it
returns { NULL, NULL } instaed of waiting.
---

diff --git a/doc/internals/api/mt_list.txt b/doc/internals/api/mt_list.txt
index 8a6bd88ae..46d86eaf7 100644
--- a/doc/internals/api/mt_list.txt
+++ b/doc/internals/api/mt_list.txt
@@ -376,6 +376,9 @@ mt_list_lock_prev(elt)
   Return    A  elt
   value:    <===>
 
+mt_list_try_lock_prev(elt)
+    Does the same thing as mt_list_lock_prev(), except if the list is
+    locked already, it returns { NULL, NULL } instead of waiting.
 
 mt_list_lock_elem(elt)
     Locks the element only. Both of its pointers are replaced by two locked
diff --git a/include/import/mt_list.h b/include/import/mt_list.h
index 86d44e517..9339d0e07 100644
--- a/include/import/mt_list.h
+++ b/include/import/mt_list.h
@@ -780,6 +780,28 @@ static MT_INLINE struct mt_list mt_list_lock_prev(struct mt_list *lh)
 	return el;
 }
 
+/*
+ * Same as mt_list_lock_prev(), except it doesn't wait if the prev
+ * is locked already, and just returns { NULL, NULL }
+ */
+static MT_INLINE struct mt_list mt_list_try_lock_prev(struct mt_list *lh)
+{
+	struct mt_list el;
+	struct mt_list missed = { NULL, NULL };
+
+	el.prev = __atomic_exchange_n(&lh->prev, MT_LIST_BUSY, __ATOMIC_RELAXED);
+	if (el.prev == MT_LIST_BUSY)
+		return missed;
+
+	el.next = __atomic_exchange_n(&el.prev->next, MT_LIST_BUSY, __ATOMIC_RELAXED);
+	if (el.next == MT_LIST_BUSY) {
+		lh->prev = el.prev;
+		__atomic_thread_fence(__ATOMIC_RELEASE);
+		return missed;
+	}
+	return el;
+}
+
 
 /* Element <el> is locked on both sides, but the list around it isn't touched.
  * A copy of the previous element is returned, and may be used to pass to