Merge patch series "assorted ->i_count changes + extension of lockless handling"

Mateusz Guzik <mjguzik@gmail.com> says:

The stock kernel support partial lockless in handling in that iput() can
decrement any value > 1. Any ref acquire however requires the spinlock.

With this patchset ref acquires when the value was already at least 1
also become lockless. That is, only transitions 0->1 and 1->0 take the
lock.

I verified when nfs calls into the hash taking the lock is typically
avoided. Similarly, btrfs likes to igrab() and avoids the lock.
However, I have to fully admit I did not perform any benchmarks. While
cleaning stuff up I noticed lockless operation is almost readily
available so I went for it.

Clean-up wise, the icount_read_once() stuff lines up with
inode_state_read_once(). The prefix is different but I opted to not
change it due to igrab(), ihold() et al.

* patches from https://patch.msgid.link/20260421182538.1215894-1-mjguzik@gmail.com:
  fs: allow lockless ->i_count bumps as long as it does not transition 0->1
  fs: relocate and tidy up ihold()
  fs: add icount_read_once() and stop open-coding ->i_count loads

Link: https://patch.msgid.link/20260421182538.1215894-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 11559c513dfb..c5c31a6f7ed5 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2218,8 +2218,21 @@ static inline void mark_inode_dirty_sync(struct inode *inode)
 	__mark_inode_dirty(inode, I_DIRTY_SYNC);
 }
 
+/*
+ * returns the refcount on the inode. it can change arbitrarily.
+ */
+static inline int icount_read_once(const struct inode *inode)
+{
+	return atomic_read(&inode->i_count);
+}
+
+/*
+ * 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)
 {
+	lockdep_assert_held(&inode->i_lock);
 	return atomic_read(&inode->i_count);
 }