vfs: elide smp_mb in iversion handling in the common case

According to bpftrace on these routines most calls result in cmpxchg,
which already provides the same guarantee.

In inode_maybe_inc_iversion elision is possible because even if the
wrong value was read due to now missing smp_mb fence, the issue is going
to correct itself after cmpxchg. If it appears cmpxchg wont be issued,
the fence + reload are there bringing back previous behavior.
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240815083310.3865-1-mjguzik@gmail.comReviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/libfs.c

@@ -2003,13 +2003,19 @@ bool inode_maybe_inc_iversion(struct inode *inode, bool force)
 	 * information, but the legacy inode_inc_iversion code used a spinlock
 	 * to serialize increments.
 	 *
-	 * Here, we add full memory barriers to ensure that any de-facto
-	 * ordering with other info is preserved.
+	 * We add a full memory barrier to ensure that any de facto ordering
+	 * with other state is preserved (either implicitly coming from cmpxchg
+	 * or explicitly from smp_mb if we don't know upfront if we will execute
+	 * the former).
 	 *
-	 * This barrier pairs with the barrier in inode_query_iversion()
+	 * These barriers pair with inode_query_iversion().
 	 */
+	cur = inode_peek_iversion_raw(inode);
+	if (!force && !(cur & I_VERSION_QUERIED)) {
 		smp_mb();
 		cur = inode_peek_iversion_raw(inode);
+	}
+
 	do {
 		/* If flag is clear then we needn't do anything */
 		if (!force && !(cur & I_VERSION_QUERIED))
@@ -2038,20 +2044,22 @@ EXPORT_SYMBOL(inode_maybe_inc_iversion);
 u64 inode_query_iversion(struct inode *inode)
 {
 	u64 cur, new;
+	bool fenced = false;
 
+	/*
+	 * Memory barriers (implicit in cmpxchg, explicit in smp_mb) pair with
+	 * inode_maybe_inc_iversion(), see that routine for more details.
+	 */
 	cur = inode_peek_iversion_raw(inode);
 	do {
 		/* If flag is already set, then no need to swap */
 		if (cur & I_VERSION_QUERIED) {
-			/*
-			 * This barrier (and the implicit barrier in the
-			 * cmpxchg below) pairs with the barrier in
-			 * inode_maybe_inc_iversion().
-			 */
+			if (!fenced)
 				smp_mb();
 			break;
 		}
 
+		fenced = true;
 		new = cur | I_VERSION_QUERIED;
 	} while (!atomic64_try_cmpxchg(&inode->i_version, &cur, new));
 	return cur >> I_VERSION_QUERIED_SHIFT;