[PATCH 2/4] kernfs: Prepare directory revisions for lockless reads

Negative dentry revalidation only needs to sample the parent directory
generation and compare it with the value recorded at lookup time.
Annotate those accesses with READ_ONCE() and WRITE_ONCE() so the
comparison can safely move outside kernfs_rwsem.

Revision updates remain serialized by kernfs_rwsem. Assert that writer
contract in kernfs_inc_rev(); the read half of the increment stays plain
because the semaphore excludes other writers.

Signed-off-by: Shakeel Butt <[email protected]>
---
 fs/kernfs/kernfs-internal.h | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index aa784b540b36..20a0cf42ba8d 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -147,20 +147,19 @@ static inline struct kernfs_node *kernfs_dentry_node(struct dentry *dentry)
 static inline void kernfs_set_rev(struct kernfs_node *parent,
 				  struct dentry *dentry)
 {
-	dentry->d_time = parent->dir.rev;
+	WRITE_ONCE(dentry->d_time, READ_ONCE(parent->dir.rev));
 }
 
 static inline void kernfs_inc_rev(struct kernfs_node *parent)
 {
-	parent->dir.rev++;
+	lockdep_assert_held_write(&parent->dir.root->kernfs_rwsem);
+	WRITE_ONCE(parent->dir.rev, parent->dir.rev + 1);
 }
 
 static inline bool kernfs_dir_changed(struct kernfs_node *parent,
 				      struct dentry *dentry)
 {
-	if (parent->dir.rev != dentry->d_time)
-		return true;
-	return false;
+	return READ_ONCE(parent->dir.rev) != READ_ONCE(dentry->d_time);
 }
 
 extern const struct super_operations kernfs_sops;
-- 
2.53.0-Meta