iversion: use atomic64_try_cmpxchg)

Use atomic64_try_cmpxchg instead of
atomic64_cmpxchg (*ptr, old, new) == old in inode_set_max_iversion_raw,
inode_maybe_inc_version and inode_query_iversion. x86 CMPXCHG instruction
returns success in ZF flag, so this change saves a compare after cmpxchg
(and related move instruction in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails, enabling further code simplifications.

The loop in inode_maybe_inc_iversion improves from:

    5563:	48 89 ca             	mov    %rcx,%rdx
    5566:	48 89 c8             	mov    %rcx,%rax
    5569:	48 83 e2 fe          	and    $0xfffffffffffffffe,%rdx
    556d:	48 83 c2 02          	add    $0x2,%rdx
    5571:	f0 48 0f b1 16       	lock cmpxchg %rdx,(%rsi)
    5576:	48 39 c1             	cmp    %rax,%rcx
    5579:	0f 84 85 fc ff ff    	je     5204 <...>
    557f:	48 89 c1             	mov    %rax,%rcx
    5582:	eb df                	jmp    5563 <...>

to:

    5563:	48 89 c2             	mov    %rax,%rdx
    5566:	48 83 e2 fe          	and    $0xfffffffffffffffe,%rdx
    556a:	48 83 c2 02          	add    $0x2,%rdx
    556e:	f0 48 0f b1 11       	lock cmpxchg %rdx,(%rcx)
    5573:	0f 84 8b fc ff ff    	je     5204 <...>
    5579:	eb e8                	jmp    5563 <...>

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

include/linux/iversion.h

@@ -123,17 +123,12 @@ inode_peek_iversion_raw(const struct inode *inode)
 static inline void
 inode_set_max_iversion_raw(struct inode *inode, u64 val)
 {
-	u64 cur, old;
+	u64 cur = inode_peek_iversion_raw(inode);
 
-	cur = inode_peek_iversion_raw(inode);
-	for (;;) {
+	do {
 		if (cur > val)
 			break;
-		old = atomic64_cmpxchg(&inode->i_version, cur, val);
-		if (likely(old == cur))
-			break;
-		cur = old;
-	}
+	} while (!atomic64_try_cmpxchg(&inode->i_version, &cur, val));
 }
 
 /**
@@ -197,7 +192,7 @@ inode_set_iversion_queried(struct inode *inode, u64 val)
 static inline bool
 inode_maybe_inc_iversion(struct inode *inode, bool force)
 {
-	u64 cur, old, new;
+	u64 cur, new;
 
 	/*
 	 * The i_version field is not strictly ordered with any other inode
@@ -211,19 +206,14 @@ inode_maybe_inc_iversion(struct inode *inode, bool force)
 	 */
 	smp_mb();
 	cur = inode_peek_iversion_raw(inode);
-	for (;;) {
+	do {
 		/* If flag is clear then we needn't do anything */
 		if (!force && !(cur & I_VERSION_QUERIED))
 			return false;
 
 		/* Since lowest bit is flag, add 2 to avoid it */
 		new = (cur & ~I_VERSION_QUERIED) + I_VERSION_INCREMENT;
-
-		old = atomic64_cmpxchg(&inode->i_version, cur, new);
-		if (likely(old == cur))
-			break;
-		cur = old;
-	}
+	} while (!atomic64_try_cmpxchg(&inode->i_version, &cur, new));
 	return true;
 }
 
@@ -304,10 +294,10 @@ inode_peek_iversion(const struct inode *inode)
 static inline u64
 inode_query_iversion(struct inode *inode)
 {
-	u64 cur, old, new;
+	u64 cur, new;
 
 	cur = inode_peek_iversion_raw(inode);
-	for (;;) {
+	do {
 		/* If flag is already set, then no need to swap */
 		if (cur & I_VERSION_QUERIED) {
 			/*
@@ -320,11 +310,7 @@ inode_query_iversion(struct inode *inode)
 		}
 
 		new = cur | I_VERSION_QUERIED;
-		old = atomic64_cmpxchg(&inode->i_version, cur, new);
-		if (likely(old == cur))
-			break;
-		cur = old;
-	}
+	} while (!atomic64_try_cmpxchg(&inode->i_version, &cur, new));
 	return cur >> I_VERSION_QUERIED_SHIFT;
 }