linux-kernel - [PATCH v2] kernfs: dont take d

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Date:   Tue, 01 Nov 2022 16:27:45 +0800
From:   Ian Kent <raven@...maw.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>
Cc:     Minchan Kim <minchan@...nel.org>,
        Eric Sandeen <sandeen@...deen.net>,
        Al Viro <viro@...IV.linux.org.uk>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH v2] kernfs: dont take d_lock on revalidate

In kernfs_dop_revalidate() when the passed in dentry is negative the
dentry directory is checked to see if it has changed and if so the
negative dentry is discarded so it can refreshed. During this check
the dentry d_lock is taken to mitigate against a possible concurrent
rename.

But if it's racing with a rename, becuase the dentry is negative, it
can't be the source it must be the target and it must be going to do
a d_move() otherwise the rename will return an error.

In this case the parent dentry of the target will not change, it will
be the same over the d_move(), only the source dentry parent may change
so the dentry d_lock isn't needed.

v2: Fix i_lock -> d_lock in patch subject and description.
    There's no change to the patch itself.

Signed-off-by: Ian Kent <raven@...maw.net>
---
 fs/kernfs/dir.c |   24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 3990f3e270cb..6acd9c3d4cff 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1073,20 +1073,30 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
 
 		/* If the kernfs parent node has changed discard and
 		 * proceed to ->lookup.
+		 *
+		 * There's nothing special needed here when getting the
+		 * dentry parent, even if a concurrent rename is in
+		 * progress. That's because the dentry is negative so
+		 * it can only be the target of the rename and it will
+		 * be doing a d_move() not a replace. Consequently the
+		 * dentry d_parent won't change over the d_move().
+		 *
+		 * Also kernfs negative dentries transitioning from
+		 * negative to positive during revalidate won't happen
+		 * because they are invalidated on containing directory
+		 * changes and the lookup re-done so that a new positive
+		 * dentry can be properly created.
 		 */
-		spin_lock(&dentry->d_lock);
+		root = kernfs_root_from_sb(dentry->d_sb);
+		down_read(&root->kernfs_rwsem);
 		parent = kernfs_dentry_node(dentry->d_parent);
 		if (parent) {
-			spin_unlock(&dentry->d_lock);
-			root = kernfs_root(parent);
-			down_read(&root->kernfs_rwsem);
 			if (kernfs_dir_changed(parent, dentry)) {
 				up_read(&root->kernfs_rwsem);
 				return 0;
 			}
-			up_read(&root->kernfs_rwsem);
-		} else
-			spin_unlock(&dentry->d_lock);
+		}
+		up_read(&root->kernfs_rwsem);
 
 		/* The kernfs parent node hasn't changed, leave the
 		 * dentry negative and return success.