The patch titled
Subject: iversion: use atomic64_try_cmpxchg)
has been added to the -mm mm-nonmm-unstable branch. Its filename is
iversion-use-atomic64_try_cmpxchg.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/iversion-use-atomic64_try_cmpxchg.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Uros Bizjak <ubizjak@xxxxxxxxx>
Subject: iversion: use atomic64_try_cmpxchg)
Date: Sun, 21 Aug 2022 21:30:11 +0200
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/20220821193011.88208-1-ubizjak@xxxxxxxxx
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/iversion.h | 32 +++++++++-----------------------
1 file changed, 9 insertions(+), 23 deletions(-)
--- a/include/linux/iversion.h~iversion-use-atomic64_try_cmpxchg
+++ a/include/linux/iversion.h
@@ -123,17 +123,12 @@ inode_peek_iversion_raw(const struct ino
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
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 *i
*/
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 *
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;
}
_
Patches currently in -mm which might be from ubizjak@xxxxxxxxx are
llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first.patch
epoll-use-try_cmpxchg-in-list_add_tail_lockless.patch
buffer-use-try_cmpxchg-in-discard_buffer.patch
aio-use-atomic_try_cmpxchg-in-__get_reqs_available.patch
iversion-use-atomic64_try_cmpxchg.patch
