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From:   Ondrej Mosnacek <omosnace@redhat.com>
To:     Alexander Viro <viro@zeniv.linux.org.uk>
Cc:     linux-fsdevel@vger.kernel.org,
        linux-security-module@vger.kernel.org, selinux@vger.kernel.org,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        Martin Pitt <mpitt@redhat.com>
Subject: [PATCH 1/2] fs: convert simple_xattrs to RCU list
Date:   Thu,  1 Sep 2022 17:26:31 +0200
Message-Id: <20220901152632.970018-2-omosnace@redhat.com>
In-Reply-To: <20220901152632.970018-1-omosnace@redhat.com>
References: <20220901152632.970018-1-omosnace@redhat.com>
MIME-Version: 1.0
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Precedence: bulk

Use the RCU list mechanism instead of a simple lock to access/modify
simple_xattrs. The performance benefit is probably negligible, but it
will help avoid lock nesting concerns for an upcoming patch.

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
---
 fs/xattr.c            | 36 ++++++++++++++++++++++--------------
 include/linux/xattr.h |  1 +
 2 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/fs/xattr.c b/fs/xattr.c
index a1f4998bc6be..fad2344f1168 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -22,6 +22,8 @@
 #include <linux/audit.h>
 #include <linux/vmalloc.h>
 #include <linux/posix_acl_xattr.h>
+#include <linux/rculist.h>
+#include <linux/rcupdate.h>
 
 #include <linux/uaccess.h>
 
@@ -1030,8 +1032,8 @@ int simple_xattr_get(struct simple_xattrs *xattrs, const char *name,
 	struct simple_xattr *xattr;
 	int ret = -ENODATA;
 
-	spin_lock(&xattrs->lock);
-	list_for_each_entry(xattr, &xattrs->head, list) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(xattr, &xattrs->head, list) {
 		if (strcmp(name, xattr->name))
 			continue;
 
@@ -1044,10 +1046,18 @@ int simple_xattr_get(struct simple_xattrs *xattrs, const char *name,
 		}
 		break;
 	}
-	spin_unlock(&xattrs->lock);
+	rcu_read_unlock();
 	return ret;
 }
 
+static void simple_xattr_free_rcu(struct rcu_head *rcu)
+{
+	struct simple_xattr *xattr = container_of(rcu, struct simple_xattr, rcu);
+
+	kfree(xattr->name);
+	kvfree(xattr);
+}
+
 /**
  * simple_xattr_set - xattr SET operation for in-memory/pseudo filesystems
  * @xattrs: target simple_xattr list
@@ -1094,11 +1104,11 @@ int simple_xattr_set(struct simple_xattrs *xattrs, const char *name,
 				xattr = new_xattr;
 				err = -EEXIST;
 			} else if (new_xattr) {
-				list_replace(&xattr->list, &new_xattr->list);
+				list_replace_rcu(&xattr->list, &new_xattr->list);
 				if (removed_size)
 					*removed_size = xattr->size;
 			} else {
-				list_del(&xattr->list);
+				list_del_rcu(&xattr->list);
 				if (removed_size)
 					*removed_size = xattr->size;
 			}
@@ -1109,15 +1119,13 @@ int simple_xattr_set(struct simple_xattrs *xattrs, const char *name,
 		xattr = new_xattr;
 		err = -ENODATA;
 	} else {
-		list_add(&new_xattr->list, &xattrs->head);
+		list_add_rcu(&new_xattr->list, &xattrs->head);
 		xattr = NULL;
 	}
 out:
 	spin_unlock(&xattrs->lock);
-	if (xattr) {
-		kfree(xattr->name);
-		kvfree(xattr);
-	}
+	if (xattr)
+		call_rcu(&xattr->rcu, simple_xattr_free_rcu);
 	return err;
 
 }
@@ -1169,8 +1177,8 @@ ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
 	}
 #endif
 
-	spin_lock(&xattrs->lock);
-	list_for_each_entry(xattr, &xattrs->head, list) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(xattr, &xattrs->head, list) {
 		/* skip "trusted." attributes for unprivileged callers */
 		if (!trusted && xattr_is_trusted(xattr->name))
 			continue;
@@ -1179,7 +1187,7 @@ ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
 		if (err)
 			break;
 	}
-	spin_unlock(&xattrs->lock);
+	rcu_read_unlock();
 
 	return err ? err : size - remaining_size;
 }
@@ -1191,6 +1199,6 @@ void simple_xattr_list_add(struct simple_xattrs *xattrs,
 			   struct simple_xattr *new_xattr)
 {
 	spin_lock(&xattrs->lock);
-	list_add(&new_xattr->list, &xattrs->head);
+	list_add_rcu(&new_xattr->list, &xattrs->head);
 	spin_unlock(&xattrs->lock);
 }
diff --git a/include/linux/xattr.h b/include/linux/xattr.h
index 979a9d3e5bfb..3236c469aaac 100644
--- a/include/linux/xattr.h
+++ b/include/linux/xattr.h
@@ -86,6 +86,7 @@ struct simple_xattrs {
 
 struct simple_xattr {
 	struct list_head list;
+	struct rcu_head rcu;
 	char *name;
 	size_t size;
 	char value[];
-- 
2.37.2