fs: allow lockless ->i_count bumps as long as it does not transition 0->1

With this change only 0->1 and 1->0 transitions need the lock.

I verified all places which look at the refcount either only care about
it staying 0 (and have the lock enforce it) or don't hold the inode lock
to begin with (making the above change irrelevant to their correcness or
lack thereof).

I also confirmed nfs and btrfs like to call into these a lot and now
avoid the lock in the common case, shaving off some atomics.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20260421182538.1215894-4-mjguzik@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 61ce251ad83e..c5c31a6f7ed5 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2227,8 +2227,8 @@ static inline int icount_read_once(const struct inode *inode)
 }
 
 /*
- * returns the refcount on the inode. The lock guarantees no new references
- * are added, but references can be dropped as long as the result is > 0.
+ * returns the refcount on the inode. The lock guarantees no 0->1 or 1->0 transitions
+ * of the count are going to take place, otherwise it changes arbitrarily.
  */
 static inline int icount_read(const struct inode *inode)
 {