[PATCH v0] kernfs: Skip kernfs_put of parent from child node

[Gaurav Kohli]

While adding kernfs node for child to the parent kernfs
node and when child node founds that parent kn count is
zero, then below comes like:

WARNING: fs/kernfs/dir.c:494 kernfs_get+0x64/0x88

This indicates that parent is in kernfs_put path/ or already
freed, and if the child node keeps continue to
make new kernfs node, then there is chance of
below race for parent node:

CPU0				         CPU1
//Parent node			         //child node
kernfs_put
atomic_dec_and_test(&kn->count)
//count is 0, so continue
					  kernfs_new_node(child)
					  kernfs_get(parent);
					  //increment parent count to 1
					  //warning come as parent count is 0
					  /* link in */
					  kernfs_add_one(kn);
					  // this should fail as parent is
					  //in free path.
					  kernfs_put(child)
kmem_cache_free(parent)
					  kmem_cache_free(child)
					  kn = parent
					  atomic_dec_and_test(&kn->count))
					  //this is 0 now, so release will
					  continue for parent.
					  kmem_cache_free(parent)

To prevent this race, child simply has to decrement count of parent
kernfs node and keep continue the free path for itself.

Signed-off-by: Gaurav Kohli <gkohli@xxxxxxxxxxxxxx>
Signed-off-by: Mukesh Ojha <mojha@xxxxxxxxxxxxxx>

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index b84d635..d5a36e8 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -515,7 +515,6 @@ void kernfs_put(struct kernfs_node *kn)
 	if (!kn || !atomic_dec_and_test(&kn->count))
 		return;
 	root = kernfs_root(kn);
- repeat:
 	/*
 	 * Moving/renaming is always done while holding reference.
 	 * kn->parent won't change beneath us.
@@ -545,8 +544,8 @@ void kernfs_put(struct kernfs_node *kn)
 
 	kn = parent;
 	if (kn) {
-		if (atomic_dec_and_test(&kn->count))
-			goto repeat;
+	/* Parent may be on free path, so simply decrement the count */
+		atomic_dec_and_test(&kn->count);
 	} else {
 		/* just released the root kn, free @root too */
 		idr_destroy(&root->ino_idr);
-- 
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