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Subject[PATCH] iversion: Use atomic64_try_cmpxchg
Date
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 <...>

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
include/linux/iversion.h | 32 +++++++++-----------------------
1 file changed, 9 insertions(+), 23 deletions(-)

diff --git a/include/linux/iversion.h b/include/linux/iversion.h
index 3bfebde5a1a6..eb5a15810169 100644
--- a/include/linux/iversion.h
+++ b/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;
}

--
2.37.1
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