From: Alexander Aring <aahringo@redhat.com>
Date: Wed, 30 Sep 2020 22:37:29 +0000 (-0400)
Subject: fs: dlm: fix race in nodeid2con
X-Git-Tag: v5.10-rc1~179^2
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=4f2b30fd9b4bd6e3620fe55786df7fc5f89ad526;p=thirdparty%2Fkernel%2Flinux.git

fs: dlm: fix race in nodeid2con

This patch fixes a race in nodeid2con in cases that we parallel running
a lookup and both will create a connection structure for the same nodeid.
It's a rare case to create a new connection structure to keep reader
lockless we just do a lookup inside the protection area again and drop
previous work if this race happens.

Fixes: a47666eb763cc ("fs: dlm: make connection hash lockless")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>
---

diff --git a/fs/dlm/lowcomms.c b/fs/dlm/lowcomms.c
index b7b7360be609e..79f56f16bc2ce 100644
--- a/fs/dlm/lowcomms.c
+++ b/fs/dlm/lowcomms.c
@@ -175,7 +175,7 @@ static struct connection *__find_con(int nodeid)
  */
 static struct connection *nodeid2con(int nodeid, gfp_t alloc)
 {
-	struct connection *con = NULL;
+	struct connection *con, *tmp;
 	int r;
 
 	con = __find_con(nodeid);
@@ -213,6 +213,20 @@ static struct connection *nodeid2con(int nodeid, gfp_t alloc)
 	r = nodeid_hash(nodeid);
 
 	spin_lock(&connections_lock);
+	/* Because multiple workqueues/threads calls this function it can
+	 * race on multiple cpu's. Instead of locking hot path __find_con()
+	 * we just check in rare cases of recently added nodes again
+	 * under protection of connections_lock. If this is the case we
+	 * abort our connection creation and return the existing connection.
+	 */
+	tmp = __find_con(nodeid);
+	if (tmp) {
+		spin_unlock(&connections_lock);
+		kfree(con->rx_buf);
+		kfree(con);
+		return tmp;
+	}
+
 	hlist_add_head_rcu(&con->list, &connection_hash[r]);
 	spin_unlock(&connections_lock);